汽车维护的原则是（）。
A、预防为主，强制维护
B、定期维护
C、日常维护
D、保持技术状况，降低故障
参考答案：A
在敞开的容器中很难把液体的（）和着火区别开。
A、瞬燃
B、闪燃
C、爆燃
D、爆闪
The Great Pyramid is said to ________by aliens. Is there a chance that this might be true? [ ]
A. be built
B. have been built
C. being built
D. built
