[求助]关于og9中的丛句和不定式问题，请大家帮忙解惑！

9.     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

请问如果D中改为they believe that is of,那么对不对？还有就是E中they believe to be是不是插入语？还是从句？为什么og说这里用to be 比用that从句要好？是不是一定这样还是要看具体的context?

有没有nn给出总结？不甚感谢！看了一下午，自己没法体会！

因为是believe，用to be表示不确定的意思