They travelled ____ the steam to find out where it had got polluted .A.along B.near C.down D.

题目

They travelled ____ the steam to find out where it had got polluted .A.along B.near C.down D.
They travelled ____ the steam to find out where it had got polluted .
A.along B.near C.down D.by
为什么选C请详解,并翻译这句话,

答案

travel down 顺流而下
他们沿着溪流顺流而下,想弄清哪里被污染了
和A 比起来,C的过程更全面,从头到尾都包括了,A 有可能是从半途沿着