The clouds _____ up the grand valley of ice _______ as the Western Cum looked thin and weak.
A.drifting......known
B.drifted....knowing
C.drifting....knowing
D.driftd...known

选A。

我们可以将这个句子拆开来看，以了解主被动关系。

The clouds _____ up the grand valley of ice _______ as the Western Cum looked thin and weak.
1、The clouds drifted up the grand valley of ice.
2、The clouds was known as the Western Cum .
3、The clouds looked thin and weak.

其中1、2两句好比是原句的成分，3句为句子主干。
拆开来后可以知道，1句中，clouds和drift是主动行为。
2句中，clouds是被know的。
非谓语中，主动行为便用-ing。被动行为便用-ed形式。

这道题的难点在于，如何划分句子成分，和非谓语的主被动用法。