Answer:
C. Mostly true -- it's mandatory, but it is certainly helpful.
Explanation:
The statement is mostly true because it will certainly help the reviewer even though it is not mandatory.
Reading a play ahead of time is helpful because it enables the reviewer to grasp an understanding of what the play is all about and helps him/her compose his thoughts well. Even though not reading it ahead of time is not mandatory as the reviewer can still do a thorough work when it's time to review.
