Question

Yesterday I was about to leave for the market ________ my boss called and asked me to go back to the office at once．

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

Answer 1

A
解析：
…be about to do sth when…固定句型，表示“……正要……此時(shí)……”。