【正确答案】 D

【答案解析】[理由] 空格内若置入(A)项them，则两句缺少连接词连接，属错误结构：
To facilitate our efficiency, management has purchased more than 50 desktop computers, many of them are to be installed in the accounting section.(×)
上句可将them改为关系代词which，形成定语从句，以修饰之前的先行词more than 50 desktop computers(50多部台式电脑)。
由上得知，凡从属从句中有数量词(如：one、two、three、four、some、many、most)，之后又有介词of时，之后均可置关系代词whom(代替人)或which(代替事或物)，以引导定语从句，修饰句中的先行词。
[例] Mr. Johnson has taught many students, some of them have become great scientists.(×，两句无连接词连接)
→ Mr. Johnson has taught many students, some of whom have become great scientists.(√)
(约翰逊先生教过许多学生，当中的一些成了优秀的科学家。)