The form in and returned within three weeks.
A. is to be filled
B. will fill
C. is to fill
D. are going to be filled
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(答案→)A. is to be filled
B. will fill
C. is to fill
D. are going to be filled
解析：时间状语within three weeks，表示将来的一段时间。be to表示将来一段时间内必定要进行的动作，而be going to表示“打算做某事”，故排除D，同时不定式的逻辑主语是form，动词形式用被动，排除B和C，故选A。