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

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

A.when

B.while

C.as

D.because

答案及解析： A

解析：

…be about to do sth when…固定句型，表示“……正要……此时……”。