40. Those who have studied English as a foreign language know _____ to become fully fluent.
(A) what a difficult job it is
(B) what difficult it is
(C) how difficult is it
(D) how a difficult job it is

Those who have studied English as a foreign language know _____ to become fully fluent. 

一個句子，就是 s 主詞  v動詞    o 受詞   補詞。 to不定詞，表前面動詞做了某事接著要去做某動作，副詞。

第一段黃色的全是形容 主詞 those，因此刪去不看。 to 不定詞 是詞容動詞用的，所以也把它刪去。

因此這句就會變成這樣而已

those(主詞)  know (動詞)  ____。
(A) what(形) a difficult(形) job(名詞) it(名詞) is      就感嘆句的用法，what 是形容詞，所以是what(形) a 形容詞+n.

      二個名詞在一起，都是關代形容的概念。後面的句子、名詞 形容前面的名詞，或更更前面的名詞。雖然甚至可以拿來形詞主詞(某些文學……)  這是說 what a +形容詞+名詞的句形而已。表多麼的之意

原句是  those know that what a difficult job (that)it is(的確是，強調之意)
(B) what difficult it is  形容詞+形容詞  在it 中間要加個名詞。what difficult直接形容代名詞不行，語意也無法表達。
(C) how difficult is it    HOW 是副詞，後面接形容詞 這正確。但同B的問題一樣，外加 HOW DIFFICULT無法直接做主詞

     而後面加 is。
(D) how a difficult job it is  how 是副詞，後面不加 a difficult 應該是 how difficult a job it is.

What+a/an+形容詞+(S+V)！

How+形容詞+S+V 

(c)應該是錯在is it ，要改成it is