10. The sign at the check-in desk says: "Passengers _______ show their tickets." A. might B. may C. must D. can 11. The doctor said to him: "You ______ exercise or you certainly have a heart attack again. A. must B. mustn't C. don't have to D. may 12. My best friend ______ big brown eyes and a round face. A. has B. does C. gets D. takes 13. Nam is such a/an ______ student. He joins many clubs in my school. A. activity B. active C. activities D. actively 14. My Uncle seldom smiles. He is always ________. A. serious B. seriousness C. series D. seriously 15. Thanh doesnt have many friends. Mike has lots of friends. Mike is ______ than Thanh A. more sociable B. more careful C. older D. bigger 16. We will save the environment if we ___________ plastic. A. reuses B. reuse C. reusing D. reused 17. We are trying to __________ the president to the party. A. pollute B. exchange C. invite D. cause 18. Why dont we __________ old school objects for candies? A. swap B. raise C. wrap D. recycle 19. They use ________ plastic bottles to keep water. A. refillable B. natural C. polluting D. charitable