题目

一门旧式大炮水平发射出一枚质量为10 kg的炮弹，炮弹飞出的对地速度是600 m/s，炮身的质量是2 t，则大炮后退的速度是________m/s；若大炮后退中所受的阻力是它重力的30%，则大炮能后退________m. 答案：3       1.5The past two years ________ a sharp rise in oil price，which adds to the burden of auto transportation industry.A．witnessed B．has witnessed C．is witnessing D．witnesses