刘振民语法改错一道题，请教

tress

Astronomers at the Palomar Observatory have discouvered a distant supernova explosion, one (that they believe is) a type previously unknown to science.
a. that they believe is
b. that they believe it to be
c. they believe that it is of
d. they believe that is
e. they believe to be of

正确答案 e

为什么a错误？they believe是否可以看做是插入语？如同 the face that I think is red一样，可以将I think省略后，不影响句子结构。one that （they believe） is a type previously unknown to science中的they believe也是一个插入语，that引导定语从句修饰one，作为supernova的同位语。各位有什么看法？

tress

bow//深受启发

fudanwyh

one that always wrong, the explosion is not a type unknown but is of a type unknown

sdcar2010

This question hinges on the use of an idom: believe something to be.  In this case, the astronomers believe a recently discovered explosion to be a new type.

For A, the word "that" is redundant since "one" already refers to the explosion.