Professor Smith，along with his assistants，on the project day and night to meet the deadlin

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A. work　　　　　　
B. working
(答案→)C. is working
D. are working
解析：该句主语是Professor Smith，故谓语动词用单数。当together with，as well as，no less than，rather than，but，except，along with，like，besides，as much as，more than...连接两个主语时，谓语动词要与前面的主语保持一致。