A 0.140 - kg baseball is dropped and reaches a speed of 1.20 m/s just before it hits the ground. It rebounds with a speed of 1.00 m/s. What is the change of the ball's momentum?
A.
0.0280 kg ∙ m/s upwards
B.
0.0280 kg ∙ m/s downwards
C.
0.308 kg ∙ m/s upwards
D.
0.308 kg ∙ m/s downwards
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【单选题】听力原文:W: Air pollution is one of the most serious problems in the world. Polluted air can make us sick or even kill us. Why isn't more done to stop it? M: Everyone wants to stop it, but it is a difficu...
A.
Most people don't think it is a serious problem.
B.
Most of our pollution is caused by things people need.
C.
The government has not spent enough money to stop it.
D.
The seriousness of pollution has not been realized by the government.