Frank was on the point of leaving ________ he noticed a letter lying on the table．

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

Frank was on the point of leaving        he noticed a letter lying on the table.

A．when        B．before       C．while        D．as

Frank was on the point of leaving ______ he noticed a letter lying on the table.

A. when　　　　 　　 B. before　　　 C. while　　　　 D. as