Many children are in the countryside when their parents flood into the cities for jobs．
A. left behind
B. left off
C. left aside
D. left out
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(答案→)A. left behind
B. left off
C. left aside
D. left out
解析：考查短语辨析。句意“当父母涌进大城市打工时，许多孩子都被留在乡下的家里了。”A“遗留，留下”；B“终端，戒除”；C“搁置”；D“漏掉，省略”。故选A。