脑部一般缺氧多长时间，人会死亡？（）
A.1～2小时
B.20分钟
C.4分钟
参考答案：C
To his parents’ great joy, all their son’s years of hard work finally ____.
A．set off
B．paid off
C．took off
D．got off
有效市场细分的条件有哪些？
