A．The five questions were not well designed.
B．Not all the questionnaires were returned.
C．Only a small number of students were surveyed.
D．Some of the answers to the questionnaire were not valid.

A．It provides them with the opportunity to watch performance for free.
B．It provides them with the opportunity to make friends.
C．It gives them the chance to do something creative.
D．It gives them a chance to enjoy modem art.