Ex1: Choose the best answer to complete these following sentences. 1.Astronauts_____ in their spaceship, but they frequently work outside now. A. were used to stay B. used to stay C. were staying D. had used to stay 2. People _____ that the earth is round before. A. were not used to believe B. used to believing C. would not use to believe D. did not use to believe 3. Tuberculosis_____ incurable before. A. use to be thought B. used to be thought C. used to think D. use to think 4. Formerly babies_____ of whooping cough. A. have died B. used to die C. would die D. both B and C 5. These city girls are used_____ in the field. A. to work B. used to lie C. would like D. to working 6. Newton_____ scientific books when a boy. A. used to read B. has read C. had read D. had been reading ( phải giải thích)