题目

设A、B、C是平面内不共线的三点,若向量=(1,1),n=(1,-1),且n·=2,则n·等于(    )A.-2                  B.2                C.-2或2                  D.0 答案：答案:B解析:∵=(1,1),n=(1,-1),∴·n=1×1+1×(-1)=0.又∵n·=2,∴n·=n·(-)=n·-n·=2.--I can't stop smoking, doctor. --For your health, I'm afraid you_______.A. may B. can C. have to D. need