.Young____he is, he has proved to be an able salesman.
选择一项：
a. as 解析：虽然还很年轻，他已经证明自己是一个有能力的销售人员。该题考查as用法。As可引导让步状语，如：Child as he is, he knows a lot. (他虽然还是个孩子，但懂得很多。)Hard as he works, he makes little progress. (虽然他工作很努力，但几乎没有进步。)Try as he might, he could not find a job. (不管怎样努力，他还是找不到工作。)
b. that
c. which
d. who