Question

complete the sentence with the best correlative conjunctions.
on earth, the most abundant greenhouse gas is __ carbon dioxide—produced by animals as they breathe—__ methane, which is produced through the decomposition of organic material.
either...nor
either...nor
not only...but also

Explanation:

To determine the correct correlative conjunction, we analyze the context. The sentence is stating two greenhouse gases: carbon dioxide (from animal respiration) and methane (from organic material decomposition). The conjunction "not only... but also" is used to connect two related and affirmative ideas, which fits here as both are greenhouse gases and the sentence is presenting them as part of the "most abundant" ones. "Either... or" implies a choice, and "neither... nor" implies a negative (not either), which don't fit the context of listing two abundant greenhouse gases.

Answer:

not only... but also