34. The average (surface temperature) of the Earth (has been risen) by (approximately) one degree (over the last) century.
我选的答案是B，有的答案是C，请解释一下。谢谢
37. Wild birds and their eggs have been (at least) incidental sources of food for humans (since) prehistoric times and they (still) are in (most of) societies.
我认为答案是D，但是有些答案是C，不知道为什么，请大侠给解释一下。 | 
