单项选择题

Why can we think of the asteroid 65 million years ago as one of our lucky stars

A．Because it extinguished dinosaurs.
B．Because it later evolved into the planet on which we live -- the earth.
C．Because it' s the asteroid' s blows that make the world as it is today and assure mankind' s success as a species.
D．Because it made thousands of species die of starvation.

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单项选择题

What's Kirkwood Gap

A．A belt located between Mars and Jupiter.
B．Different regions in the asteroid belt.
C．The mysterious gap between different regions of the asteroid belt.
D．Ceres, a ball about 637 miles in diameter.

单项选择题

According to the second theory, why did the solar system clear everywhere except in the asteroid belt

A．The asteroid belt was too far away from the sun.
B．In the asteroid belt, the gravity of Jupiter was strong enough to balance the opposing pull of the sun.
C．The solar system prevented fragments from accreting into their own planets.
D．The asteroids are leftovers from the days of creation.