Redwood trees are the tallest plants on the earth,reaching heights of up to 100 meters.They are also known for their longevity,typically 500 to 1000 years,but sometimes more than 2000 years.A hundred million years ago,in the age of dinosaurs,redwoods were common in the forests of a much more moist and tropical North America.As the climate became drier and colder,they declined to a narrow strip along the Pacific coast of Northern Califomia. The trunk of redwood trees is very strong and usually forms a single straight column.It is covered with a soft bark.This bark can be pretty thick,well over two feet in the more mature trees.It gives the older trees a certain kind of protection from insects,but the main benefit is that it keeps the center of the tree harmless from moderate forest fires because of its thickness.This fire resistant quality explains why the giant redwood grows to live that long.While most other types of trees are destroyed by forest fires,the giant redwood actually prospers because of them. Moderate fires will clear the ground of competing plant life,and the rising heat dries and opens the ripe fruit of the redwood,releasing many thousands of seeds onto the ground below. New trees are often produced from shoots,little baby trees,which form at the base of the trunk.These shoots grow slowly,fed by the root system of the "mother'' tree.When the main tree dies,the shoots are then free to grow as full trees,forming a "fairy ring" of trees around the original tree.These trees,in turn,may give rise to more shoots,and the cycle continues. Why do redwood trees grow to live that long according to the passage? A) They have heavy and straight tree trunks. B) They are properly watered and catered to. C) They are more resistant to fire damage than other trees. D) They produce many young trees to maintain their life cycle.
C

Standing in front of the "Chilkoot Trailhead" sign for 15 minutes, I didn't take one step. I was afraid of bears. That's what was keeping me from walking the trail alone. I had hiked through black-bear country back home in Colorado, but now I was in Alaska, home to the fierce brown grizzly bear. Fortunately, a friendly couple arrived and asked me to take their picture. We started talking and soon were walking the trail together. I was relieved. Ron and Carol were from Alaska and they taught me how to stay safe in bear country. I had finally started my hike along the famous Chilkoot Trail, where more than 115 years ago, thousands of gold miners had walked. I was writing a book about the Klondike Gold Rush, and I wanted to follow in the footsteps of the miners to know what it was really like. Ron, Carol, and I walked through the forest. As I was looking around at the beautiful scenery in the forest, I tripped   over a big tree root and injured my ankle  . Fortunately, my ankle felt better in the morning, and we followed the trail. We went to bed early that night because the following day would be the toughest hike of all. At dawn, we started hiking up the steep  , green valley. Icy glaciers covered the mountains above. Soon we were as high as the clouds and could see only 20 feet in front of us. Finally, we reached the top of the Chilkoot pass and saw a Canadian flag moving in the heavy wind. Hours later, we arrived at our campground. The following morning, my ankle was feeling fine, and my fear of bears had mostly disappeared. I said good-bye to Ron and Carol and walked alone for the next two days. Without seeing any bears the entire trip, I finally arrived at Bennett Lake. I rode the old train down to the town of Skagway and checked into my hotel. While relaxing in my comfortable bed in the hotel, I thought of the miners from the Klondike Gold Rush. When the miners reached Bennett Lake, they had to cut down trees, build boats, and travel 500 miles down dangerous rivers and lakes. Then, when they finally... When crossing the Chilkoot Trail, the author   _  . A) got help from two local miners B) once tried to take a boat trip C) hurt himself by accident D) was almost killed by a brown bear
C

As if there weren't enough of them in the world already, scientists have succeeded in cloning flies. The alike fruit flies are the first insects ever cloned, says the Canadian team that created them. The question everyone asks is why anyone would want to clone flies in the first place. She hopes that the insects , which are very easy to experiment with , will help to adjust the cloning process in other animals and even in humans, where the technique is being researched to aid production of therapeutic   stem cells. In cloning, the DNA-containing nucleus of an adult cell is injected into an egg whose own nucleus  has been removed. At the moment, the majority of cloned mice, sheep and other animals die before birth. It is thought this is because the adult DNA is not properly 'reprogrammed'. Using flies, researchers might discover genes that are important for this reprogramming, and that have matches in other animals. That is because it is relatively easy in flies to knock out the function of a single gene and then attempt cloning with these cells, which will test whether that gene is important. If such genes are confirmed, then in theory cloned mammalian embryos  might be grown in selected solutions that change the activity of those genes to improve the technique's success rate. Although some groups have attempted to clone insects before, Lloyd says this is the first time it has been successful. The team used a slight difference on the standard cloning process: they transferred nuclei taken from embryos rather than fully-grown adult cells. They sucked several nuclei (pl)out of developing fly embryos, and injected them into a fertilized  fly egg. From over 800 early attempts, they cloned five adult insects, the group reports in the journal Genetics. The scientists cloned flies because  _  . A) there are fewer and fewer flies in the world B) flies are easy to find and cheap to use C) they can gain some experience before cloning other animals D) it is safe to clone flies

C