请看下面这道题：
Mr Smith has bought a little house in the country, around _________ some green trees. A. which is B. it is C. which are D. them are
【分析】此题【分mso-hansi-font-family: 'Times New Roman'">容易误选A或B，将A、B中的 which 和 it 误认为是其后句子的主语。其实，此题的最佳答案为C，around which are some green trees 是一个由around which are some green t; FONT-FAMILY: 'Times New Roman'">“介词+which”引出的非限制性定语从句，而在该从句中，主语是 some green trees，around which 是表语，所以句子谓语应用are，而不是 is。请看类例：Among these people was a foreigner from Europe. 他们当中有一个来自欧洲的 style="FONT-SIZE: 10.5pt; FONT-FAMILY: 'Times New Roman'">Among these peopl老外。(句子主语是 a foreigner，among these people 是表语，所以谓语动词用单数 was)On the top of the tree were some birds. 树顶上有一些鸟。(句子主语为 some birds，on the top of the tree ，是表语，所以谓语动词用复数 were) The police found a large box under his bed, in which were some sex books. 警察在他的床底下找到一只大箱子，里面装的是一些黄色书籍。(in which were some sex books 为非限制性定语从句，在该从句中，in which 是表语，some sex books 是主语，所以谓语动词用复数were)