Trees absorb these gases. Global temperatures are rising. Many variables affect global temperatures. Many human activities produce these gases.
I gave my students this multiple choice question: Many scientists are convinced that an increase of certain gases in the atmosphere is affecting global temperatures. What makes this hypothesis difficult to prove?

Only one answer is correct among these 4 choices:
- Trees absorb these gases.
- Global temperatures are rising.
- Many variables affect global temperatures.
- Many human activities produce these gases.

Could you tell me which one is correct?

Many variables affect global temperatures.