Historically, global climate
a. has always been the same.
c. has varied greatly over time.
b. has varied slightly over time.
d. none of the above.

Option B, has varied greatly over time

Explanation:

The climate change on planet earth is a long and continuous process which is ongoing till today. It is believed that some four billion years ago, the atmosphere of earth was made up of radical ions and also the earth was excessively hot. After this phase the life evolved on earth and the climate also improved. After several years of its formation, the earths climate again took a drastic turn when the ice age came and every thing just got frozen. This occurred during the Pleistocene Epoch period some 2.6 million years ago.

Later on , the ice age also passed and now a days temperature is rising year by year due to global warming.

Due to these drastic conditions it can be said that the climate of earth changed greatly over time.

it is either c or d and i know for sure it is not a or b