现代汽车的制动系，一般都增设（），以避免或减少车轮被抱死现象。
参考答案：电子控制的制动防抱死装臵 制动力分配调节装臵
石油分馏可以得到汽油、煤油、柴油等．由此推断石油属于（ ）
A．化合物
B．混合物
C．纯净物
D．单质
-The two astronauts had been trained for long before theyflew to space.
-Oh, they____quite a lot of difficulties. [ ]
A. might go through
B.may go through
C. should have gone through
D. must have gone through
