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?