边际技术替代率是劳动投入与资本投入相互替代的比率，即等成本线的斜率。
参考答案：错
The children ______ a PE class on the playground when it suddenly began to rain. [ ]
A. have
B. are having
C. had
D. were having
The road sign is easy to read, for the words______well.[ ]
A．come out
B．stand out
C．go out
D．keep out
