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Unsolved Mysteries About Planet Earth
♦Mystery 1 ： Where did all the water come from?
Water covers 70 percent of Earth’s surface and earns it the nickname “the blue planet”. And where did it come from? The most popular scientific theory states that the H2() arrived in the form of several violent asteroids (小打星)filled with ice. Another suggests that the water actually has been around since Earth's for matioa However it happened, though, it ^certainly worked out well for Earth’s life forms.
♦Mystery 2 ： What about all the oxygen?
Another thing is the planet s oxygen. Tiny creatures released oxygen as a waste product, filling the atmosphere with it After that, the level of oxygen in Earth’s atmosphere went wildly up and down until it finally calmed down around 540 million years ago. Since then, it's remained at about the breathable level we experience today. But what caused it to be suddenly steady?
♦Mystery 3： What caused the Cambrian (寒武纪）Explosion?
The Cambrian Explosion refers to the explosion of complex life that occurred on the Earth about 540 million years ago. Before then，life had consisted mostly of bacteria. But at the beginning of the Cambrian period, complex creatures began developing at a rate never before seen* Suddenly» life forms had brains» eyes, and bones. Most creatures alive today can trace their blood back to the Cambrian period
♦Mystery 4： Will we ever be able to predict earthquakes?
We still haven't been able to come up with a way to accurately predict earthquakes. We can certainly try, but our current technology cannot predict them exactly. We know that earthquakes start when rocks crack underground and send earthquake waves toward the sur- face，but we haven’t figured out why that happens，or how to predict it.
21. What was created later by creatures on the Earth?
A.Water. B. Bacteria. C.Oxygen. D. Ice.
22. Which of the following talks about the Earth’s advanced species?
A. Mystery 1. B. Mystery 2. C. Mystery 4. D. Mystery 3.
23. What do we know about Mystery 4?
A. It is a matter that has extremely practical significance.
B. It will be solved in the near future.
C. It is the most difficult to solve of the four.
D. It is the oldest mystery of the four.
