Given a scientific question, generate a correct answer to it.

Input: Consider Input: Carboxylic acids have an acidic hydrogen atom, but esters do not. what do esters have in place of an acidic hydrogen atom?

Output: hydrocarbon group

Input: Consider Input: What do stored fats provide our body with for later use?

Output: energy

Input: Consider Input: The first living cells may have evolved around how long ago?
Output: 4 billion years