阅读理解

阅读下面短文，从每题所给的四个选项(A，B，C和D)中，选出最佳选项。

　　Many people believe the glare(炫目的光) from snow causes snowblindness. Yet with dark glasses or not they find themselves suffering from headaches and watering eyes and even snowblindness when exposed to several hours of “snow light”.

　　The United States army has now determined that glare from snow does not cause snowblindness in troops in a snow-covered country. Rather a man's eyes frequently find nothing to focus on(聚集) in a broad space of snow-covered without-grass land. So his gaze continually moves and jumps back and forth over the entire landscape in search of something to look at. Finding something hour after hour the eyes never stop searching and the eyeballs become tired and the eye muscles ache. Nature makes up for this discomfort by producing more and more fluid(流体) which covers the eyeball. The fluid covers the eyeball in increasing quantity until it makes eyes difficult to see dearly and the result is total even though for a short time snowblindness.

　　Experiments led the Army to a simple method of overcoming this problem. Scouts(侦察兵) ahead of the troops are trained to shake snow from evergreen bushes creating a dotted line as they cross completely snow-covered landscape. Even the scouts themselves throw lightweight dark-colored objects ahead on which they can focus too. The men following can then see something. Their gaze is arrested . Their eyes focus on a bush and having found something to see stop searching the snowblanketed landscape. By focusing their attention on one object at a time the men can cross the snow without becoming hopelessly snowblind or lost. In this way the problem of crossing a continuous white land is overcome.

1．To prevent snowblindness caused by the strong light from snow wearing glasses or not ________.

[　　]

A．depends on whether the snow is white enough

B．makes no difference

C．makes much difference

D．depends on whether the snow is thick

2．When the eyes are tired tears flow out ________.

[　　]

A．to clear the vision

B．to make the eyes stop searching

C．to make the vision unclear

D．to produce more and more liquid

3．Snowblindness can be avoided ________.

[　　]

A．by moving one's gaze back and forth

B．by walking ahead and keeping looking around

C．by making up for the discomfort of one's eyes

D．by providing the eyes with something to focus on

4．What is the probable meaning of the underlined part “Their gaze is arrested” (in paragraph 3)?

[　　]

A．They get something to look at.

B．They can only look at one spot.

C．Their eyes are clear.

D．They can't see freely.