题目

I don't thinkdifficult for a Chinese student to master a foreign language within five years A.that          B.it        C.too      D.very 答案：B如图，有一条长为a的斜坡AB，它的坡角∠ABC=45°，现保持坡高AC不变，将坡角改为∠ADC=30°，则斜坡AD的长为（　　）A．aB．2aC．2a