计费的对象有哪些？
参考答案：
主叫用户计费、入中继计费、出中继计费、被叫用户计费、下级局用户计费、呼叫卡号计费、第三方计费。
— Hello, I ____ you ____ in London, how long have you been here?
— I came here last week. [ ]
A. don't know; were
B. didn't know; were
C. haven't known; are
D. hadn't known; are
假定在一定的社会背景下，社会的边际消费倾向C为0.5，则乘数K为( )
A．8
B．6
C．4
D．2
