过卷扬限制器〔又称上升极限位置限制器）的功用是当吊钩升到上限极限位置时，它开始动作，切断电源.以确保安全。（）
A、正确
B、错误
参考答案：A
—Why was he late for school yesterday?
— He overslept. By the time he got to the bus stop, the bus ________ already ______.
A．has; left
B．had; left
C．was; leaving
D．have; left
为保证筛粉效率，给料要（）。
