Do you know___ he could have dealt with, if he had not passed the test. Awhat Bhow Cwhere Dwhy

Do you know___ he could have dealt with, if he had not passed the test. Awhat Bhow Cwhere Dwhy

这是一个由What引导的宾语从句，且what在这里有俩个角色一是引导从句二是再从句中作deal with 的宾语。正因为deal是不及物动词所以它宾语之间必须接介词with（可以直接接宾语的叫及物动词不可以直接接宾语的叫不及物动词）。而how不可以在从句中作宾语，它一般引导方式状语从句。

deal是不及物动词但with 是一个介词，deal 与with合用就相当于及物了，所以with后面需要宾语，即选what.

从结构上说，what to do
how to do it

这道题do you know后面是空，也就是空白处需要一个名词（或相当于名词的成分），这几个选项可以引导名词性从句的只有what.

再有，如果句子变成：He could have dealt with a lot of barriers, if he had not passed the test. 如果对a lot of barriers提问，就变成了这道题目了。