在卡诺循环中，当热源与冷源温差为0时，卡诺循环热效率为（）。
A、1
B、0
C、0.5
D、0.1
参考答案：B
—It's difficult for the village children to cross the river for school.
—I think a bridge __________over the river. [ ]
A. should be built
B. is being built
C. has been built
D. was built
