instruction:
Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
question:
Oxygen gas is given off by plants.
answer:
Which gas is given off by plants?


question:
Simple carbohydrates can be classified in terms of chain length.
answer:
Which carbohydrates can be classified in terms of chain length?


question:
Fruit fly and brine shrimp hox genes have evolved independently for 400 million years long.
answer:

Fruit fly and brine shrimp hox genes have evolved independently for how long?