The mother didn't know ________ to blame for the broken glass as it happened while she was out.
A.whatB.howC.whenD.who在线课程D
名词性从句的引导词中，what, who, how, when等词在从句中均充当成分，在从句中作主语、宾语及表语时用what，指人用who，作状语表方式、时间及地点时用how, when, where，引导词+不定式结构相当于简略式从句。需指出的是，that引导名词性从句时引导词在从句中不作任何成分，what只能引导名词性从句，不能引导定语及状语从句。