Here are the correct answers for the underlined words or phrases:

**29.** A. Mr. Smith **is** going to buy **(B)** new Japanese cars, doesn’t he?
**Correction:** A. (is) should be corrected to "isn't he?" for agreement.

**30.** I met a lot of interesting **(B)** people while I was studying **(D)** Ho Chi Minh City.
**Correction:** D. (at) should be corrected to "in."

**32.** The country and the city have advantages and **(31)** disadvantages. People in the country live in more beautiful surroundings. They enjoy **(32)** **(B)** quietness, and can do their work at their **(33)** **pace** because no one is in a **(34)** **(A)** hurry and ready to help them **(35)** **(D)** if they need it. Their life, however, can be **(36)** **(B)** boring, and they may be isolated, which is a serious problem **(37)** (A).

**Correction for 31:** A. (B) should be corrected to "disadvantages."
**Correction for 32:** B. (quietness) is the correct usage, but it can also be "quiet."

**38.** The city has all the services that the country lacks, but it **(38)** has a lot of **(B)** disadvantages. Cities are often polluted. They not **(39)** **(D)** only have polluted air but also have noisy streets. Everyone is always in a hurry and this **(40)** **(D)** means that people have no time to get to know each other and make friends.

**Correction for 39:** D. (not only) should be corrected for agreement with "but also."
**Correction for 40:** D. (means) is acceptable but could be considered "implies."

Overall, focus should be on correcting subject-verb agreement, prepositions, and conjunctions to form clearer sentences.