题目

若曲线x2+y2+a2x+(1-a2)y-4=0关于直线y-x=0的对称曲线仍是其本身，则实数a等于(    )A.±                 B.±              C.或          D.或 答案：解析：将曲线整理成标准形式得,由题意，圆心在直线y-x=0上，∴,∴a=±.答案：B--- What on earth are the police busy with?--- They have been investigating how the train crash __________. A．came outB．came acrossC．came aboutD．came up