2019年GMAT考试语文模拟试题：句子改错（5）
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Quasars, at billions of light-years from Earth the most distant observable objects in the universe, believed to be the cores of galaxies in an early stage of development.
(A) believed to be
(B) are believed to be
(C) some believe them to be
(D) some believe they are
(E) it is believed that they are