prep中的一道语法题求解释！

yyy2013

Although the first pulsar, or rapidly spinning collapsed star, to be sighted was in the summer of 1967 by graduate student Jocelyn Bell, it had not been announced until February, 1968.

A. Although the first pulsar, or rapidly spinning collapsed star, to be sighted was in the summer of 1967 by graduate student Jocelyn Bell, it had not been announced until February, 1968.  

B. Although not announced until February, 1968, in the summer of 1967 graduate student Jocelyn Bell observed the first pulsar, or rapidly spinning collapsed star, to be sighted.  

C. Although observed by graduate student Jocelyn Bell in the summer of 1967, the discovery of the first sighted pulsar, or rapidly spinning collapsed star, had not been announced before February, 1968.  

D. The first pulsar, or rapidly spinning collapsed star, to be sighted was observed in the summer of 1967 by graduate student Jocelyn Bell, but the discovery was not announced until February, 1968.  

E. The first sighted pulsar, or rapidly spinning collapsed star, was not announced until February, 1968, while it was observed in the summer of 1967 by graduate student Jocelyn Bell.  

纠结于cd之间，答案是d，但觉得to be sighted的用法挺奇怪的，求解释！

mnimi

C中省略的主语是pulsar,而不是后半句的the discovery