Astronomers at the Palomar Observatory have discovered a distant supernova explotion, one that they belive is a type previously unknown to science. A-D全保留了that. A that they belive is B that they believe it to be C that they believe it is of D that belive that is E正确选项 they belive to be OG解释： 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 大全解释： 前面one that they believe is , 正常顺序they believe that one is...变成one+that定语从句, 因为one不作主语，所以一般省that, 后面believe +that宾语从句,在定语从句中出现的宾语从句，that一般也省，所以应该是one they believe is，不过这里believe宾语从句不如believe to do好。