为预防胎儿发生神经管畸形，孕期应补充叶酸，开始补充的时间应在

A．孕前至少1个月

B．孕后1个月

C．孕后2个月

D．孕后3个月

E．分娩前1个月

She had never been to the Great Wall before.Today she saw it        the first time.      [ ]

A. at    

B. for    

C. in    

D. by