The law requires all cars () for safety and efficiency.

单项选择题The weight of something is another way of describing the amount of force exerted on () by gravity.

单项选择题Prior () his departure, he wrote a letter to his mother.

单项选择题The owner of the store promised to give the money back to the traveller（）his return.

A.at
B.from
C.by
D.on

单项选择题Only after I left the place that how ugly and dishonest the people there had been.

单项选择题When the clouds moved in, the moon was no longer () sight.

单项选择题Going () sleep for a long time makes me feel very tired.

单项选择题If I had () in business, I might be a very wealthy man today.

单项选择题As a party member, one should try to his duty.

单项选择题W expects to register its biggest Cyber Monday yet, coming off a record Black Friday weekend online.()

单项选择题The only games（）I play are football and tennis.

A.who
B.which
C.what
D.when