The manager was worried about the press conference his assistant in his place but ,luckily

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A. gave
B. gives
C. was giving
D. had given
解析：C【命题动向】考查动词时态。
解析一：根据下句but luckily，可知之前经理担心他的助手正在代替他召开的媒体招待会，故用过去进行时。
解析二：句意为“经理当时很担心助理替他主持的记者招待会，所幸的是一切进展顺利”，根据语境排除B项；结合上下文应是当时在举行的记者招待会，故选C项。