品牌定位知觉图的空白部分（）等于市场机会。
A.一定
B.不
C.基本
D.不一定
参考答案：D
He spent thirty minutes in finishing his homework.
A．It took him half an hour to finish his homework.
B．It takes him half an hour to finish his homework.
C．It cost him half an hour to finish his homework.
D．He spend half an hour to on finishing his homework.
访问控制的三要素是什么？按访问控制策略划分，可分为哪几类？按层面划分，可分为哪几类？
