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

Example Input: The main advantage of urea is its very low toxicity.
Example Output: The main advantage of urea is its very low what?

Example Input: The human body has as many as two trillion lymphocytes, and lymphocytes make up about 25% of all of leukocytes.
Example Output: The human body has as many as two trillion lymphocytes, and lymphocytes make up about 25% of all of what?

Example Input: Cracks forming in a boulder of the following can be caused by weathering.
Example Output:
Which of the following can be caused by weathering?