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

Watching children, particularly      they don’t know you are doing so, is really a pleasure.
    A. as       B. while      C. when      D. since

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

[　　]

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

A．as

B．when

C．while

D．since

A．as

B．when

C．while

D．since