下列不属于聚合工艺安全控制的基本要求的是（）
A、紧急切断系统
B、反应釜温度和压力的报警和连锁
C、液位监控
参考答案：C
By the time Jack returned home from America, his son __________ from college.
A．graduate
B．has graduated
C．had graduated
D．had been graduated
富氧环境是指空气中氧的体积百分比高于（）%。
