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

OG的解释中说明，A，B选项中的that“should be deleted”，好像表明有that必须除去，但是搜索了前面所有人的讨论，that都只是可以省略，没有说必须省略。这个题根据believe to be 和of可以判断正确答案是E。但是，到底这个that错误是正确性还是有效性呢？可不可以根据这个来判断错误选项？