新生儿期是指出生后脐带结扎开始至满（）
A.24天
B.28天
C.30天
D.42天
E.60天
参考答案：B
当供给同样的氮气和空气时，降低可燃成分浓度，氮气比空气用量要（）。
A、多
B、少
C、一样
33．Not until he called the secretary three times ______ that the manager went to an important meeting．
A．did he tell
B．he told
C．was he told
D．he was told
