【正确答案】 D

【答案解析】短语辨析。fall apart破裂，破碎，如：Their marriage finally fell apart．他们的婚姻最终破裂了。fall behind落后，如：After her time in hospital，Jenny's parents are afraid she has fallen behind educationally．珍妮的父母担心女儿出院后学习成绩会落后于其他同学。fall down摔倒，崩溃，失败，如：She fell down the stairs．她从楼梯上摔下来。The plan fell down because it proved to be too expensive．这项计划因费用过高而告吹。fall off数量或质量下降，如：Audience figures fell off during the second series of the programme．该节目第二集的观众人数下降了。根据句意，D最合适。