sc - astronomers

[Xbond]

Hi there,

I don't understand how to attack this SC how to resolve
What is the concept behind; A clear explanation is requested.


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

OA is E

[[email protected]]

believe to be...idiom...

E...