prep-p2-18 不明白啊,请指教

prep-p2-18

18.   (25381-!-item-!-188;#058&002338)

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.

答案

我有个疑问,D中的to be sighted 和was observed 意思不重复么?而且A哪里错了呢?