用表示阿伏加德罗常数的数值，下列说法正确的是   

A．1 mol Na₂O₂晶体中共含有个离子

B．0.1 molAlCl₃完全水解转化为氢氧化铝胶体，生成0.1N_A个胶粒

C．常温常压下，和混合气体含有个氧原子

D．电解法冶炼铝时每转移个电子，阳极生成氧气

It is difficult for us to learn a lesson in life ______ we've actually had that lesson.[ ]

A. when

B. after

C. since

D. until