根据保护层与被保护层之间垂距H，当H等于或小于（）m时为近距离保护层。
A、10
B、10～50
C、50
参考答案：A
______ you keep trying till the end, you will never know how strong you are!
A．Although
B．If
C．Unless
D．Because
We're considering students' request ______the school cafeteria should provide more kinds of food．
A．how
B．what
C．that
D．whether
