If the new safety system _______ to use, the accident would never have happened.
had been put
were put
should be put
would be put
We human beings should be very careful with what we do with nature, _____power is beyond our understanding.
()I found the lecture hard to follow because it ____ when I arrived.
I haven’t finished my report, but I promise I ______ it by the end of the week.
It's a custom in China to have some tea or other drinks before the meal ________.
—What’s all that noise about outside? —Oh, I forgot to tell you. The road to the railway station ______.