D slightly distorts the meaning of the sentence.
"as they did", this phrase ambiguously indicate that once, long time ago, we had seen the Quasars during the formation.
While according to the A, no evidence is applied to prove that we had once seen the formation. We are seeing that for this moment, not "once".
-- by 会员 2008115061 (2010/8/24 21:07:22)