矿石
参考答案:
凡是地壳里面的矿物集合体,在现代技术经济水平条件下,能以工业模从中提取国民经济所必需的金属或矿物产品的,就叫做矿石。
若单摆的摆长不变,摆球的质量由20g增加为40g,最大摆角由4°减小为2°,则单摆振动( )
A.频率不变,振幅不变
B.频率不变,振幅改变
C.频率改变,振幅不变
D.频率改变,振幅改变
____ from his appearance, he must be from the north.
A.Judging
B.Judge
C.Judged
D.To judge