Given a context passage, generate a question from the passage such that its answer is shortest continous span from the passage.
Q: Some minerals are very useful. An ore is a rock that contains minerals with useful elements. Aluminum in bauxite ore (Figure 1.1) is extracted from the ground and refined to be used in aluminum foil and many other products. The cost of creating a product from a mineral depends on how abundant the mineral is and how much the extraction and refining processes cost. Environmental damage from these processes is often not figured into a products cost. It is important to use mineral resources wisely. Geologic processes create and concentrate minerals that are valuable natural resources. Geologists study geological formations and then test the physical and chemical properties of soil and rocks to locate possible ores and determine their size and concentration. A mineral deposit will only be mined if it is profitable. A concentration of minerals is only called an ore deposit if it is profitable to mine. There are many ways to mine ores. Aluminum is made from the aluminum- bearing minerals in bauxite. Surface mining allows extraction of ores that are close to Earths surface. Overlying rock is blasted and the rock that contains the valuable minerals is placed in a truck and taken to a refinery. As pictured in Figure 1.2, surface mining includes open-pit mining and mountaintop removal. Other methods of surface mining include strip mining, placer mining, and dredging. Strip mining is like open pit mining but with material removed along a strip. These different forms of surface mining are methods of extracting ores close to Earths surface. Placers are valuable minerals found in stream gravels. Californias nickname, the Golden State, can be traced back to the discovery of placer deposits of gold in 1848. The gold weathered out of hard metamorphic rock in the western Sierra Nevada, which also contains deposits of copper, lead, zinc, silver, chromite, and other valuable minerals. The Underground mining is used to recover ores that are deeper into Earths surface. Miners blast and tunnel into rock to gain access to the ores. How underground mining is approached from above, below, or sideways depends on the placement of the ore body, its depth, the concentration of ore, and the strength of the surrounding rock. Underground mining is very expensive and dangerous. Fresh air and lights must also be brought into the tunnels for the miners, and accidents are far too common. The ores journey to becoming a useable material is only just beginning when the ore leaves the mine (Figure separated out of the ore. A few methods for extracting ore are: heap leaching: the addition of chemicals, such as cyanide or acid, to remove ore. flotation: the addition of a compound that attaches to the valuable mineral and floats. smelting: roasting rock, causing it to segregate into layers so the mineral can be extracted. To extract the metal from the ore, the rock is melted at a temperature greater than 900o C, which requires a lot of energy. Extracting metal from rock is so energy-intensive that if you recycle just 40 aluminum cans, you will save the energy equivalent of one gallon of gasoline.
A: 
when valuable minerals are taken from stream gravels, this is called