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?

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?

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?


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