[求助]OG9

9.       Astronomers at the Palomar Observatory
have discovered a distant supernova explosion, one that they believe isa 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(answer:e)
我的问题是为什么ETS给出的解释说THAT是多余的? ONE THAT THEY BELIEVE...是个同位语从句吗?

我搜了, 但是没有讨论到我问的这个问题