Circle the best option to complete each sentence: 13. The inhabitants wrote a letter of complaint to the local ............ about the noise from the factory.A. author B. authorities C. authority D. authorized 14. Using ................. transport is a good way to reduce traffic jams and air pollution. A. common B. private C. public D. train 15. I will be ................... if she manages to sell that motorbike at high price. A. surprise B. surprised C. surprising D. to surprise 16. Unless you understand, I .......... explain it again to you.A. am B. was C. Will D. would 17. If you know where she lives, please let me ......................................... A. know B. knew C. known D. to know 18. If he ............. a student, he will get a discount.A. is B. was C. Were D. will be 19. ........................ you are interested in this film, don’t go to see it at any cost. A. If B. Unless C. Because D. So 20. If you like that book, I will give it .......... you as my present.A. for B. from C. at D. to 21. I won’t go unless you ............... me back the money. A. to pay B. paid C. paying D. pay (ko cần giải thích đâu)