SC 1000 # 145

winonawu

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


为什么one 和 that 不能同时出现么？
A 和D好像没什么区别吧。。。？
我选的D。。。。
求助大家了！