Tom was about to close the windowhis attention was caught by a bird.A. when　　　B. ifC. and　

问题补充：

Tom was about to close the windowhis attention was caught by a bird.

A. when

B. if

C. and

D. till

答案：

(答案→)A. when

B. if

C. and

D. till

解析：考察固定句式。固定句式be about to do sth when…正在做某事，在这时发生了意料之外的事情。本句中的when是一个并列连词，表示在那时。句意：Tom正要关窗户，在这时她注意到了一只鸟。根据句意说明A正确。