Oskar Pfungst discovered that Hans performed badly in the math work when ______.

A．The survey received a low rate of return.
B．The sample selected was not representative.
C．The questionnaires were too difficult.
D．The results of the survey were against the public opinion.

A．Children are unfamiliar with each other’s problems.
B．Children are deprived of the opportunity to enter the adult world.
C．There are too many segregated schools.
D．Boys and girls do not study together and fail to know each other.