环阀式防气泵，能够（）。
A.泵的效率不变
B.改变泵的效率
C.提高泵的效率
D.降低泵的效率
参考答案：C
Barbara is easy to recognize as she's the only one of the women who _____ evening dress. [ ]
A. wear
B. wears
C. has worn
D. have worn
氧枪喷头损坏的原因和停用标准是什么？
