请问D中那个to be是怎么回事

lllmmm

全划线
(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.

Tobelogical

http://forum.chasedream.com/GMAT_SC/thread-570680-1-1.html

lllmmm

谢谢！