（）闸板只能用于空井情况下的关井，该闸板禁止在井内有钻具、套管的情况下关井。
A、全封
B、半封
C、剪切
参考答案：A
Schools across China are expected to hire 50,000 college graduates this year as short-term teachers, almost three times the number hired last year, ____ reduce unemployment pressures.
A．help
B．to have helped
C．to help
D．having helped
