A “check” line is（）.

A.a safety line attached to a man working over the side

B.used to measure water depth

C.used to slow the headway of a barge

D.used to measure the overhead height of a bridge

雪期是指从当年初雪日到次年终雪日的天数。读某区多年平均雪期等值线图，回答下列问题。

导致该区域雪期等值线数值自南向北变化的主要因素是（）

A．地形

B．纬度

C．洋流

D．降水