当我们用力快速地挥手时，可是还是听不到挥手的声音，是因为：（   ）

A．响度太小

B．音调太低

C．振动幅度不大

D．根本不振动

We should consider the students' request _____ the school library provide more books on popular science. [ ]

A. that

B. when

C. which

D. where