Tom was about to close the window his attention was caught by a bird.
A. when
B. if
C. and
D. till
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(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。