英汉互译。

I study English by listening to cassettes. (听录音带)

                       by studying with a group. (                )

                       by watching English programs on TV. (                )

                       by taking part in English classes after school. (                )

I study English ___________________________ (读英文杂志)。

                      ___________________________ (读教科书)。

                      ___________________________ (认真完成作业)。

手动起重机用的钢丝绳，安全系数取（）。

A、3.5

B、4.5

C、5.5

D、8