单选题
Some surveys show that a lot of people believe that the glare from snow causes snow-blindness. Yet, wearing dark glasses or not, they find themselves suffering from headaches and watering eyes, and even snow-blindness, when exposed to several hours of "snow light". The United States Army has now determined that glare from snow actually does not cause snow-blindness in troops in snow-covered country. Rather, a man's eyes frequently find nothing to focus on in a broad expanse of a wide snow-covered territory. His gaze, in consequence, continually shifts and jumps back and forth over the entire landscape in search of something he can look at. Finding nothing to focus on, hour after hour, the eyes never stop searching, and the eyeballs will become sore and eye muscles will ache. Nature reduces this uncomfortable feeling by producing more and more fluid which covers the eyeball. The fluid covers the eyeball in increasing quantity until vision becomes increasingly unclear. The result may be total, even though temporary, snow-blindness. Experiments led the Army to a simple method of overcoming this problem. A small group of soldiers will be sent ahead of a main body of troops. They are trained to shake snow from evergreen bushes, creating a dotted line as they cross completely snow-covered landscape. Even these soldiers themselves throw lightweight, dark colored objects ahead, on which they, too, can focus. The men following them can then see something. Their gaze is arrested. Their eyes focus on a bush and, having found something to see, stop searching the snow-blanketed landscape. By focusing their attention on lone object at a time, the troops can cross the snow-covered wilderness without becoming hopelessly snow-blind or lost. In this way the problem of crossing a solid white territory is overcome.
单选题
Dark glasses have proved to be ______ in overcoming headaches, watering eyes and blindness caused by the glare from snow.