题目

Not only his children but also the old man ________ himself with welfare projects. A . is concerned B . concerns C . are concerned D . concern 答案：B直线(3－a)x＋(2a－1)y＋7＝0与直线(2a＋1)x＋(a＋5)y－6＝0互相垂直，则a值是(　　) A．－　 　B.　 　C.　 　D.