题目内容：

    Strange ice rings in Siberia’s Lake Baikal have puzzled scientists for decades, but now the mystery apparently has been solved. The answer: the rings are caused by warm, circular currents of water under the ice, called eddies (漩涡).

The eddies’ strong currents melt the ice at the edge, but weaker ones keep the center frozen. “Results of our field surveys show that there are warm eddies that circulate in a clockwise (顺时针的) direction under the ice cover,” said Alexei Kouraev, a hydrologist at the University of Toulouse, in a NASA statement. “In the eddy center, the ice does not melt — even though the water is warm — because the currents are weak,” he said. “But on the eddy boundary, the currents are stronger, and warmer water leads to rapid melting.”

During field work, Kouraev and his colleagues drilled holes near ice rings and set sensors capable of measuring the temperature and salinity (盐浓度) of the water to a depth of 700 feet. They found that the water in the eddies was 2 to 4 ℉ warmer than the surrounding water.

Most of the rings appear in March or April and have a width of about 3 to 4 miles — too big to recognize from the ground but easily seen from satellites above. Some rings were ephemeral while others lasted for weeks or even months.

Lake Baikal is the world’s largest and deepest freshwater lake, according to Gizmodo. It’s home to many varieties of fish not seen anywhere else in the world, and even a population of freshwater seals.

Researchers are still investigating what causes Baikal’s eddies but think it’s likely because of wind patterns, rivers that flow into the lake and the shape of the lake’s coastline and bottom.

1.Which word can best replace the underlined word “ephemeral” in paragraph 4?

A.Steady. B.Current.

C.Fundamental. D.Temporary.

2.Why is the ice in the center of ice rings frozen while that at the edge is melted?

A.Because the center of ice rings is 2 to 4℉ warmer than the edge.

B.Because the currents in the center are less powerful.

C.Because it is too wide to be recognized or melted.

D.Because water in the center circulates in a clockwise direction.

3.Which of the following statements about Lake Baikal’s eddies is TRUE?

A.They are caused by hot and circular currents under the ice.

B.The ice in the center of Lake Baikal’s eddies melts faster than that of the edge.

C.Researchers are uncertain about the exact reason of the eddies’ existence.

D.Lake Beikal’s eddies can only be measured at the depth of 700 feet.

4.What maybe the best title for the passage?

A.What’s causing the giant ice rings in Siberia?

B.The mystery of the eddies' in the Lake Baikal uncovered.

C.The largest and deepest freshwater lake — Lake Baikal.

D.Why does the edge of an ice ring melt faster?