从降低成本和减少环境污染的角度考虑,制取硝酸铜最好的方法是( )
A.铜和浓硝酸反应
B.铜和稀硝酸反应
C.氧化铜和硝酸反应
D.氯化铜和硝酸银反应
答案:C
用括号内所给单词的适当形式填空
1. It's five o'clock. The students __________(play) football on the playground.
2. You __________(not pass) the exam if you don't work hard enough.
3. The baby is sleeping. Could you please stop __________(make) so much noise?
4. This machine __________(use) for cleaning the house.
5. The man spoke very slowly. He wanted me __________(understand) what he said.
电灯悬挂于两墙之间,如图所示,使接点A上移,但保持O点位置不变,则A点上移过程中,绳OA的拉力( )
A.逐渐增大
B.逐渐减小
C.先增大,后减小
D.先减小,后增大