题目

已知平面向量a=(3，1)，b=(x，－3)，且a⊥b，则x等于( )A.3                              B.1                              C.－1                           D.－3 答案：思路分析：由3x+1×(－3)=0,得x=1.答案：BMary wrote an article on       the team had failed to win the game.A.whyB.whereC.whatD.which