—How is the weather?
—It's _________.[ ]
A. rain
B. wind
C. suny
D. rainy
答案：D
Under perfect competition, a firm will experience zero long term economic profit when:（）
A. price is less than average total cost.
B. MR is greater than ATC.
C. MC = ATC = MR = price.
运行中的电流互感器二次（）是十分危险的。
A、短路
B、通路
C、开路
