1 . Unsolved Mysteries About the 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 water came from several violent asteroids (小行星) filled with ice. Another suggests that the water actually has been around since Earth’s formation. However it happened, though, it’s 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 forms that occurred on 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 seen before. Suddenly, life forms had brains, eyes, and bones. Most living creatures 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 surface, but we haven’t figured out why that happens, or how to predict it.

1. Which of the following is true according to the text?

A．Earth is called the blue star because of water coverage.

B．The level of oxygen has been steady all the time.

C．Complex creatures had rapid development after the Cambrian period.

D．We know nothing about predicting earthquakes.

2. Which of the following is true about Mystery 4?

A．It is the oldest mystery of the four.

B．It will be solved in the near future.

C．It is the most difficult mystery to solve.

D．It is a matter that is of practical significance.

3. Where is this text most likely from?

A．A diary.	B．A geography book.
C．A novel.	D．A magazine.