求助sat语法题！！！(Unlike) several decades ago, today's librarians

求助sat语法题！！！
(Unlike) several decades ago, today's librarians teach students to evaluate the accuracy and objectivity of online resources in addition to helping them find particular books.
A原句
B Unlike them of
C Unlike those of
D Contrary to
E Contrary to those
答案是C
请帮忙解释一下其他几个选项为什么不对。 尤其是B选项 。正确答案的those到底指什么？