首先,你的題應(yīng)該糾正為
______as he is ,he knows a lot.
A.A young boy B.Young boy
as作“盡管”講,引導(dǎo)讓步狀語從句,從句是倒裝句,要將表語或狀語放在句首.例如：
Proud as these nobles are,he is afraid to see me.這些貴族盡管很傲慢,他卻害怕見我.
Hard as he studied,he did not pass the exam.盡管他努力學(xué)習(xí),但這次考試他還是沒有及格.
Small boy as he is,he knows quite a lot.盡管他還是個小孩子,但是卻懂得很多.（注：在這種句子中,作表語的名詞前不加冠詞.）
所以答案是B.