题目

 Dave is always            , never making a decision about anything in a hurry.        A. modest               B. curious              C. cautious      D. outgoing 答案：C如图，Rt△OAB的直角边OA在y轴上，点B在第一象限内，OA=2，AB=1，若将△OAB绕点O按顺时针方向旋转90°，则点B的对应点B′的坐标是    ．