Given a scientific passage and an answer, generate a question for the given answer.

[Q]: Passage: Over 99 percent of all species that ever lived on Earth have gone extinct. Five mass extinctions are recorded in the fossil record. They were caused by major geologic and climatic events. Evidence shows that a sixth mass extinction is occurring now. Unlike previous mass extinctions, the sixth extinction is due to human actions. Answer: 99%
[A]: What percentage of all species that ever lived on earth have gone extinct?

[Q]: Passage: Large viruses were once parasitic cells inside bigger host cells. Over time, genes needed to survive and reproduce outside host cells were lost. Answer: parasitic
[A]: Large viruses began as what type of cells inside bigger host cells?

[Q]: Passage: Unlike ethers, esters have a carbonyl group. Unlike carboxylic acids, esters have no acidic hydrogen atom; they have a hydrocarbon group in its place. Answer: hydrocarbon group
[A]:
Carboxylic acids have an acidic hydrogen atom, but esters do not. what do esters have in place of an acidic hydrogen atom?