直输机稀油站内油泵出口有（）。
A.安全阀
B.止回阀
C.针形阀
D.闸阀
参考答案：B
—Excuse me, is this your ring?
—Yes.______. [ ]
A. is it
B. it's
C. it is
D. it isn't
价格在经济中的作用是什么？
