Question 1
Earth's most important internal heat source is/are ________.
◦ coal seams
◦ planetary collisions
◦ solar winds
◦ radioactivity melting
Question 2
The bodies of our solar system began forming about ________ billion years ago from an enormous cloud of minute rocky fragments and gases.
◦ 8
◦ 4
◦ 5
◦ 2