阅读下列短文，根据短文内容，从题中所给的A、B、C、D四个选项中，选出最佳答案。

Though oceans cover about 71 percent of the earth’s surface, there’s still a lot we don’t know about them. Only about 20 percent of the seafloor has been mapped, and the remaining 80 percent is still a secret to us. Some scientists even think that 91 percent of sea animals have yet to be discovered.

So why do we know so little about the deep sea? Well, getting to those depths is very difficult and requires high technology that simply didn’t come into being for most of human history. The first navigable submarine（可驾驶的潜水艇）, for example, was built in 1620, and it couldn’t go more than five meters below the surface. In fact, for a long time many scientists didn’t even think that there could be any life at depths greater than 550 meters and didn’t see much need to explore（探索） the deep sea. However, this changed in 1862 when life was discovered as far down as 945 meters below sea level with the help of a digging machine. Ten years later, around 4, 700 species（物种） unknown before from the ocean floor were discovered.

Since then, high technology has been developed to help explore the deep sea, including satellites. But why is deep-sea exploration still so difficult? Well, at greater depths it is freezing（极冷的）, everything is completely dark, and the pressure can be over 1, 000 times greater than that on the surface.

Though we have the difficulties, it’s important that we understand the seas. They help control the earth’s temperature and produce half of the oxygen（O2） in the air. They can also provide us with information about climate change. Our future depends on learning more about our oceans, and it is possible for us to really understand what is ongoing below the surface.

（1）Why are some numbers listed in Paragraph 1?

A．	To introduce how important oceans are.
B．	To discuss why sea animals have yet to be found out.
C．	To explain that oceans cover a large area of the earth.
D．	To show that we have only a little understanding of oceans.

（2）What caused the change of exploration into the deep sea?

A．	High technology.	B．	Unknown species.
C．	A pleasant environment.	D．	Valuable information.

（3）What does the underlined word “They” in the last paragraph refer to?

A．

Sea animals.

B．

The seas.

C．

The satellites.

D．

Many scientists.

（4）What can we learn from the text?

A．	It is difficult to build a navigable submarine.	B．	The seas have a strong influence on our future.
C．	The seas have no lives at a depth of 550 meters.	D．	It is easier to go into the deep sea than we think.

（5）What could be the best title of the text?

A．	Hidden Treasures in Deep Sea	B．	Unlocked Secrets of Deep Sea
C．	Deep-Sea Exploration: Into the Freezing Depths	D．	Deep-Sea Exploration: The Past and the Future