For the last 20 years, we ______ significant changes in the world of science and technology.
A. witness B. have witnessed C. witnessed D. are witnessing

Without your help, I _______ the technical problem with my computer the other day. A. wouldn’t solve B. couldn’t have solved C. could solve D. can’t solve

It is raining heavily with rolls of thunder. We _______ such a terrible thunderstorm. A. would never see B. had never seen C. have never seen D. never see

The headmaster has decided that three lecture halls ______ in our school next semester. A. will build B. will be building C. are being built D. will be built

Students will not be allowed into the exam room if they ______ their student cards. A. didn't produce B. don’t produce C. produced D. hadn’t produced

We could have caught the last train, but we ______ five minutes late. A. were B. have been C. would be D. are

______ a few more minutes, we could have finished the task. A. Unless we had B. If we had had C. If we have D. If we had

The theory of relativity ______ by Einstein, who was a famous physicist. A. was developed B. is developed C. develops D. developed

“Could you turn off the stove? The potatoes ______ for at least thirty minutes.” A. boiled B. were boiling C. are boiling D. have been boiling

“What _______ if the earth stopped moving?” A. happened B. would happen C. happens D. will happen