Once lost,-----.

Once lost,such a chance might never come again是Once it is lost,such a chance might never come again主句是such a chance might never come again其主语是a chance,而once it is lost这里的It也是指这个chance,前后两个句子是相同的主语,因此可以省略掉it is,或者说省略掉the chance isA。it is hard to get such a chance again once it is lost。主句主语是to get such a chance 要得到这么一个机会,是to do sth。跟后面的it (指chance) 是不一样的主语,这时候it is 不能省略掉B。to get such a chance will be defficult once it is lost 道理跟上面的一样,前面的主语是to do sth。C。one can nerver get such a chace again once it is lost。这里的主句说的是一个人不能再得到这么一个机会,反正说的主语也不是chance。综上,就是选D了。