请问这里该选哪个词好呢？

Then the sound of the engine started up again, setting them___, for they knew as the men were leaving, they had narrowly escaped being captured.

A at ease B at length C at leisure D at work

可以分析一下这个句子的结构吗？尤其是seting them …… 作什么成分呢？

a

set somebody at sase:是他们安心、放心； 现在分词短语起状语作用

A

可以理解为伴随状语

这个题老师讲过，应该是完形填空中的一个空。