Index fossils help scientists estimate the age of a rock because index fossil species only existed for a relatively short time. What happened to the species that are now used as index fossils?
OPTIONS:
- They became extinct.
- They changed their diets.
- They hid in marine sediments.
- They migrated to new environments.

They became extinct.


Water at 20°C in an uncovered pan is evaporating very slowly. What could be done to the water to make it evaporate more quickly?
OPTIONS:
- Cover it.
- Heat it.
- Place it in the dark.
- Put salt in it.

Heat it.


A balloon filled with helium gas is set free and starts to move upward. Which of the following best explains why the helium balloon moves upward?
OPTIONS:
- The density of helium is less than the density of air.
- The air resistance lifts the balloon up.
- There is no gravity acting on the helium balloons.
- The wind blows the balloon upward.


The density of helium is less than the density of air.