According to the passage, what are astronomers doing so we are better prepared for a possible asteroid strike?
A. building early-warning systems
B. equipping rockets with nuclear bombs
C. compiling a list of asteroids
D. studying the craters created by asteroids
查看答案
Which method of deflection wouldn't work well ona rotating asteroid?
A. exploding a nuclear bomb nearby
B. hitting it with a spacecraft
C. pulling it using gravity
D. landing on it and pushing it
What might change the course of Apophis so that it hitsthe Earth within the next 30 years
A. Its own gravity
B. a spacecraft's gravity
C. the Earth's gravity
D. a comet's gravity
在齿顶圆和齿根圆之间，规定一直径为d的圆，作为计算齿轮各部分尺寸的基准，称为。
微信支付 
支付宝支付