用例的职责通常分配给（）的对象
A.发送消息
B.接收消息
C.发送和接收双方
D.分析类
参考答案：D
The travel costs ____ 2,000 dollars.
A．reach as many as
B．are added up to
C．come to
D．add up
纯焦冶炼时，炉顶煤气是风量的（）倍。
A.1.21
B.1.3
C.1.35～1.37
D.2.0
