求教！！

coocooper

Astronomers at the palomar observatory have discovered 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

请问为什么不可以选A啊。。答案说A错在believe is的连用，但是这里they believe不是跟着that的么，那么句子就是one is a type previously unknown to science.啊。。这么用不可以么。

孤独的长跑者

believe to be of 的系表结构比 believe + 从句 的结构要清晰简洁，虽然在这句话里还多一个单词。