It was three years_______ he came back

The teacher urged that all the students ________ their papers carefully.[ ]

ne can answer the question _______ wecan complete the task in time, do you know— Yes, and this is because we don’t knowthe way _______ the scientist used last time．

I am going to the Great Wall next week.— _______.

Must I turn off the gas after cooking—Of course. You ________ never be careful enough with that.