It's about _______ from our school to my home.
A：twenty minutes' walk
B：twenty minute's walk
C：twenty minutes' walks
D：twenty-minute-walk
一架飞机水平地匀速飞行．从飞机上每隔1秒钟释放一个铁球，先后共释放4个．若不计空气阻力，则四个球（ ）
下列计算正确的是（ ）
637×2=____
