这题没什么人问，想来是比较简单，但是我就是错了，希望NN们拉我一把，看看我的分析对不对

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）

(E) they believe to be of

OG的解释是：

Choice E is best. The pronoun that in A and B should be deleted, since the pronoun one is sufficient to introduce the modifier and the sentence is more fluid without that. In B and C, it and that it are intrusive and ungrammatical: the idiom is “believe x to be y.” In the context of this sentence, the infinitive to be is more appropriate than the limited present-tense is in referring to an event that occurred long ago but has been discovered only recently. Finally, A, B, and D lack of
   and so illogically equate this particular explosion with the whole class of explosions to which it belongs: it is not a type but possibly one of a type.

此句中,one 引导同位语从句.

原句可理解为a distant supernova explosion that they believe to be one of  a  type...