请帮忙分析一道冠词选择题,要合理解释的,我选B

请帮忙分析一道冠词选择题,要合理解释的,我选B19.Tom is ▁▁most hardworking student,but he isn’t▁▁most hardworking in our class.A.the; the B.a; the C.the; a D.a; a