A．Practically every river has an upper, a middle, and a lower part.
【译文】但凡是河流，都有上游、中游和下游。
B．The purposes of his journey were both military and political.【译文】这次旅行既有军事上的目的，又有政治上的目的。
C．Know the true value of time; snatch, seize, and enjoy every moment of it.
【译文】知道时间的真正价值。才能争取每一刻，抓住每一刻，享受每一刻。
D．It must be morning now, because the birds are singing.
【译文】一定是天亮了，因为鸟儿在歌唱。

A．平均值
B．有效值
C．最大值
D．幅值

A错误
B正确