OG10 183 一个关于指代的问题

183. Archaeologists in Ireland believe that a recently discovered chalice, which dates from the eighth century, was probably buried to keep from being stolen by invaders.
(A) to keep from
(B) to keep it from
(C) to avoid
(D) in order that it would avoid
(E) in order to keep from

答案是B 我选的是A 我不明白的是 这里面一定要加指代it的么，前面那个which只是个插入结构，was不就是chalice的谓语么？ 这里为什么要加代词？离的太远了？