如图，在Rt△ABC中，∠ACB=90°，CD⊥AB，垂足为D，AB=c，∠A=α，则CD长为（ ）
读一读，选词填空。
A. thin B. soft C. hard D. thick
You'd better put your heart into study. You see, the ________ you are, the ______ progress (进步) you'll make.
