that why都不行as, which 非限定性定语从句：由as, which 引导的非限定性定语从句，as和which可代整个主句，相当于and this或and that。As一般放在句首，which在句中。As we know, smoking is harmful to one's health.The sun heats the earth, which is very important to us.典型例题 1）Alice received an invitation from her boss, ___came as a surprise.A. itB. thatC.
