OG9。么时候省略that。

wretdgdh

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

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.

为什么这个that要省略掉呢？

另外，E这个选项太完美了，但是如果E是"they believe was of" 或者"they believe that was of"谁更好呢？