对于动量守恒定律的理解有以下几种表达,指出哪种表达是正确的( )
A. 系统的动量守恒,则动能当然守恒
B. 质点作匀速圆周运动,其动量守恒
C. 几个质点组成的孤立系统,所受合外力为零,但由于质点之间有相互作用,因此,系统的动量不守恒
D. 不计空气阻力,抛体在水平方向上动量守恒,在竖直方向上动量不守恒
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一弹簧不遵守胡克定律，弹簧弹力F与伸长量x的关系为F=52.8x+38.4x2，求：（1）将弹簧从定长x1=0.5m拉伸到x2=1.0m，弹簧弹力所做的功为（）
A. 31J
B. 21J
C. 10.5J
D. 15.5J
What tube moves food from your throat to your stomach and is posterior to the trachea?
A. Esophagus.
B. Larynx.
The pancreas produces 2 hormones: ____.
A. epinephrine and insulin
B. melatonin and glucagon
C. insulin and glucagon
D. glucagon and norepinephrine
The two main(biggest) chambers of the heart are called ____.
A. atrium
B. ventricles
C. veins
D. arteries
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