The professor to walk out of the office when one of his students came in.
A. was about to
B. was going to
C. would
D. was to
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(答案→)A. was about to
B. was going to
C. would
D. was to
解析：考察固定句式be about to，将要做某事。句意：当他的一位学生进来时，这位教授即将走出他的办公室。选项BCD缺少谓语动词，故选A。