这是一篇克漏字，请问选项为什么要选which？
为什么不能选those呢？
Kizhi is an island on Lake Onega in Karelia, Russia, with a
beautiful collection of wooden churches and houses. It is one
of the most popular tourist attractions in Russia and a United Nations
Educational, Scientific, and Cultural Organization (UNESCO)
World Heritage Site.
The island is about 7 km long and 0.5 km wide. It is surrounded
by about 5,000 other islands, some of ______ are just rocks sticking
out of the ground.
(A) them (B) that (C) those (D) which
谢谢!