推煤机冷却系统的冷却介质有液体、空气和固体三种。（）
参考答案：错
People have come to understand _____ prevents so many Americans from being as happy as one might expect.
A．that it is why
B．what does it that
C．why it does that
D．what it is that
春秋战国时期，铁器的使用和牛耕的推广强化了周王室的统治。( )
