请教OG 9

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

此题答案到是选出来了,可是不明白划线that前面的one 是起什么作用,而且在很多场合可以见到,定语从句修饰one,请牛人指教.