Trees absorb these gases. Global temperatures are rising. Many variables affect global temperatures. Many human activities produce these gases.
I am hesitating between 4 options to answer the following question, which option should I choose?
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?
Possibilities:
- Trees absorb these gases.
- Global temperatures are rising.
- Many variables affect global temperatures.
- Many human activities produce these gases.

Many variables affect global temperatures.