磨矿机的充填率通常为（）
A、20—30％
B、30—40%
C、40—45%
D、40—50%
参考答案：D
He studied hard at school when he was young, ___________leads to his success in his later life.
A．what
B．which
C．that
D．so that
根据图4-22回答下面问题。
图4-22为甲、乙、丙、丁四个企业：1983-1988年利润增长状况曲线图。(以1982年为100%)
从1983年起，持续增长的企业是（）。
A．甲
B．乙
C．丙
D．丁
