3+8=____
①有点有趣____ ②来自____ ③砍倒____
④be lost____ ⑤be in great danger____ ⑥演奏乐曲____
⑦象牙制品____ ⑧好运的象征____ ⑨for a long time____
—架飞机水平匀速飞行,从飞机上每隔1s释放一个铁球,先后共释放4个,若不计空气阻力,则4个铁球( )