Sure, here are the correct answers to each of the questions:

Question 6: You __________ go to the dentist's before your toothache gets worse.
                    A. ought B. may C. should D. have
**C. should**

Question 7: He used to __________ his living by delivering vegetables to city hotels.
                    A. earn B. gain C. get D. receive
**A. earn**

Question 8: London is a large city. Its__________ is smaller than Tokyo or Shanghai.
                    A. populate B. popular C. populous D. population
**D. population**

Question 9: Her illness was __________ than we had thought first.
                   A. much serious B. more serious C. seriouser D. most serious
**B. more serious**

Question 10: John wishes he ___________ an astronaut when he grows up.
                   A. were B. will be C. had been D. would be
**A. were**

Question 11: After they ______ their breakfast, they ______ shopping yesterday.
                   A. have/go B. had had/went C. had had/go D. had/had gone
**B. had had/went**

Question 12: Trains stop here in order to _______ passengers only.
                   A. pick up B. get off C. get up D. pull in
**A. pick up**

Question 13: If it __________ tomorrow, we will stay home and watch TV.
                   A. will rain B. rained C. would rain D. rains
**D. rains**

Question 14: His house looks very large and beautiful. It is _________ house.
                  A. a seven-rooms B. seven room C. a seven-room D. seven rooms
**C. a seven-room**

Question 15: __________ Christmas Eve, people often have parties at midnight.
                  A. In B. At C. On D. For
**C. On**

Question 16: Jamie was excited ________________ to sit still for long.
                  A. enough B. too C. very D. so
**B. too**

Question 17: Don't forget _______ the door before you go to bed.
                  A. locked B. to lock C. locking D. lock
**B. to lock**

Question 18: Nga loves ducks, __________ ?
                  A. doesn't she B. does Nga C. does she D. doesn't Nga
**A. doesn't she**

Question 19: A good stereo system doesn't have to cost ______ and ______. You can find one for a reasonable price.
                  A. a hand - a leg B. an arm - a leg C. a finger - a toe D. a hand - a foot
**B. an arm - a leg**

Question 20: I dropped my phone and it broke. __________, I need to either borrow yours or buy a new one.
                     A. Because B. However C. Therefore D. Furthermore
**C. Therefore**

Question 21: The __________ language in Vietnam is Vietnamese.
                 A. nationally B. nationality C. nation D. national
**D. national**