Tom was about to close the window his attention was caught by a bird.A. whenB. ifC. andD.

问题补充：

Tom was about to close the window his 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：考查固定搭配。be about to do sth. when...“正要做某事时，这时某事发生了”。句意“汤姆正要关窗户时，他的注意力被一只鸟吸引了。”故选A。