31. Historians combine social, economical, and political events of a country into a general story, and explain how they have shaped the course of the nation.

= it has (pronoun refers to a story)

32. In the early 17th century, Korea, which it had seen the undesirable effects of colonization, protected itself from outside influences, and became a ‘closed’ country.

= which (omit it)

33. Doctors are saying that brain exercises may delay the onset of Alzheimer’s disease, so they do not address the root cause of the disease.

= but

34. Dormant for 400 years, Mount Sinabung unexpectedly erupted in August 2010, sending huge clouds of gaseous smoke and ash into the sky and to cause thousands of people to flee their homes.

= causing

35. A new study comparing organic strawberries with conventionally grown ones reveal that organic strawberries taste much better and are more nutritious.

= reveals

36. Crocodiles do not chew their food, but they swallow it in large chunks which is then broken down in the stomach.

= which are

37. A student majoring in accounting must to earn an average of C for all courses, including not only accounting courses but also history.

= earn (modals + bare infinitive)

38. Found in Tanzania, the three-millions-year-old fossils were estimated to be the oldest human remains to be discovered.

= three-million-year-old (adjective cannot be plural)

39. Translations should be done careful so that the accuracy of the original manuscript is preserved.

= carefully

40. The computer company Dell, known for it’s environmental policies such as banning the export of e-waste to developing countries, ranked┬áno 1 in Newsweek’s 2010 Green Rankings.

= its (pronoun without comma, if it uses comma, it means “it is”