Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

[EX Q]: An acorn growing into an adult tree will take the longest time to observe.
[EX A]: Which of these will take the LONGEST time to observe?

[EX Q]: Competive exclusion princple explains why two different species cannot occupy the same niche in the same place for very long.
[EX A]: What explains why two different species cannot occupy the same niche in the same place for very long?

[EX Q]: Oxygen has increased in the atmosphere throughout the history of the earth.
[EX A]:
What has increased in the atmosphere throughout the history of the earth?