请教og第九题？

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

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.

og的解释看的不是很理解，one＋they believe。。好像不是同位语的结构，但有不能确定，而且og的解释说可以more fluid 也不是行能理解。难道真的要凭语感吗？

the sentence is more fluid without that.

这就是表明gmat会尊重和考察一些语言习惯吧