单项选择题

What caused photosynthesis to stop

A．Thousands of species died of starvation.
B．Earthquake and tidal waves.
C．Massive volcanic eruptions and forest fires.
D．Tons of dust and pulverized rock blasted into the atmosphere and blocked out sunlight.

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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.

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.