33. His writing is so confusing that it's difficult to make out ___ it is he is trying to express.
A. that B. how C. who D. what
答案：D.
翻译：他的书写是如此的不清楚,以至于很难分辨清他试图表达（的东西/意图是）什么.
解释：
1. 此句是一个典型的so..that..句型,that后面引导的是结果状语从句.
2. 此结果状语从句中,make out后面是一个宾语从句,且该从句中的it is后面却是表sth的表语,当然答案只能选用D what.
而he is trying to express是定语从句,其关系词做express的宾语,所以被省略.