We know _____America is great not because it is perfect but because it can always be made better and _____ the unfinished work of perfecting the union falls to each of its citizens.

A．不填；不填

B．that ; that

C．that ; 不填

D．不填；which

易燃气体气瓶的首次充装或定期检验后的首次充装，未经置换或抽真空处理的气瓶不能充装。