___he kept thinking about was how to carry out the plan without delay A.which B.that C.How D.All 为什么不能用which,这里面which与all有什么区别呢
是不是which,that不能做主语和表语(除了定语从句之中),而what可以,which只能用于定语从句之中?
which无法单独引导名词性从句,这个解释比较靠谱~