Question

Watching children, particularly ________ they don't know you are doing so, is really a pleasure.

1. A.
   as
2. B.
   when
3. C.
   while
4. D.
   since

Answer 1

B
从句意中了解到是表时间的。when既可以跟延续性动词，也可以跟短暂性动词，且句中用否定，最佳答案为B。