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
题目分析：主语是quasars，这个词是复数（不是因为这个词加了s而是因为后面告诉我了这个词汇是objects），根据语义，不能用主动，因为这个词做不出believe这个动作，只能是被动，并且不能加they，them了，因为已经有quasars了，所以选B，并且B用了are，与主语保持了一致！
