阅读理解。

     Locations along the southern and eastern shores of the Great Lakes are famous for the tremendous

amounts of snow they receive. When the snow starts to fall, people start tossing out the phrase "lake-

effect snow".

     Primarily a late-autumn and early-winter phenomena, lake- effect snowstorms are instigated by the

movement of cold, arctic air over the relatively warm waters of the Great Lakes. Ideally, the temperature

difference between the lake water and the overrunning air should be at least 20 degrees in order for cloud

and subsequent snow to form.

     As the cool air crosses the waters of the Great Lakes, the lowest levels of the atmosphere begin to

warm and pick up moisture. This newly warmed layer is lighter than the cold air above it, so it starts to

rise. As the modified air continues to climb higher into the atmosphere, it encounters much colder

temperatures.

     This cooler air forces the moisture to condense into water droplets and ice crystals, forming cloud.

After this process repeats itself a number of times, the cloud becomes weighed down and is forced to

precipitate (凝结而下降) in the form of snow.

     Out of all the meteorological factors (气象因素) that determine snowfall intensity, the most important

might be the direction of the wind. If the wind direction is running perpendicular (垂直的) across the lake,

there won't be ample time for cloud to develop.

     However, if the wind runs parallel to the length of the lake, cloud should form without a hitch. The

longer the cold air travels over the lake, the more moisture it is able to accumulate, which in turn leads to

greater snowfall totals.

     The highest annual lake-effect snowfall totals are found across the U. P. of Michigan, northwestern

Pennsylvania and the far southwestern and upstate sections of New York. In these locations, which are

all located along the southern or eastern shores of the Great Lakes, recording over 100 inches of snow in

a winter season is a common occurrence.

1. What is the purpose of the first paragraph?

A. To tell how much snow the Great Lakes have.

B. To introduce the topic of the text.

C. To show when the lake-effect snow appears.

D. To tell what the Great Lakes are famous for.

2. What's the first important factor in the intensity of snowfall?

A. The length of the lake.

B. The direction of the wind.

C. The relatively warm waters of the lake.

D. The strength of the wind.

3. Why can northwestern Pennsylvania see heavy snowfall?

A. It's far from the sea. 

B. It's far from the north.

C. It's near the Great Lakes.

D. It has many mountains.

4. What's the text mainly about?

A. How the lake-effect snow affects life.

B. Where the lake-effect snow happens.

C. How the lake-effect snow forms.

D. How the phrase "lake-effect snow" comes.