TASK DEFINITION: Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
PROBLEM: If two populations do not mate and produce fertile offspring, separate species result.

SOLUTION: If two populations do not mate and produce fertile offspring, what results?

PROBLEM: Humans have 23 pairs pairs of chromosomes.

SOLUTION: How many pairs of chromosomes do human have ?

PROBLEM: After an infection has been brought under control and most cytotoxic t cells die off, a few remain as memory cells.

SOLUTION:
After an infection has been brought under control and most cytotoxic t cells die off, a few remain as what?