TASK DEFINITION: Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
PROBLEM: The pupil is the name of the black opening in the eye that lets light enter.

SOLUTION: What is the name of the black opening in the eye that lets light enter?

PROBLEM: The refrigerator door contains iron of the following statements best explains why magnets usually stick to a refrigerator door.

SOLUTION: Which of the following statements best explains why magnets usually stick to a refrigerator door?

PROBLEM: The acidity of a solution is typically assessed experimentally by measurement of its ph.

SOLUTION:
The acidity of a solution is typically assessed experimentally by measurement of its what?