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参考解析：
刷刷题刷刷变学霸
举一反三
【单选题】层次模型的数据结构是______。
A.
以记录类型为结点的树
B.
以记录类型为结点的有向树，图中有且仅有一个结点无父结点
C.
以记录类型为结点的有向图
D.
以记录类型为结点的有向树，图中结点有且仅有一个父结点
【单选题】(2011•南京第一次模拟)—I hear you________at Smith’s.—Yes，I________there for about three months.
A.
work；had been working
B.
worked；was working
C.
are working；have been working
D.
worked；have worked
【单选题】(2010•南京第二次模拟)I admire my English teacher. I can remember very few occasions ________she stopped working because of ill health.
A.
that
B.
when
C.
where
D.
which