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

答案应是B。不过我觉得时态不太对，应该用过去时比较妥当，因为in an early stage of

development表述的是过去一个时间段。虽然相比其他而言，这个选项更好