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单项选择题

Long hours at the screen can cause aching eyes, blurred vision and headaches, experts say. In fact, eyestrain surpasses even wrist pain as the top physical complaint among heavy computer users.
When peering into the computer screen, the eye"s focusing muscle is at constant tension. Like any muscle, it feels sore when overloaded. And when someone spends a lot of time looking at something close, the eye muscle can get stuck on the near-focus setting and have difficulty relaxing, leaving the person temporarily nearsighted, a condition called accommodative spasm(痉挛) that can last seconds or hours.
This is why after looking at a monitor all day, some computer workers complain that their distance vision is bluffed when they attempt to drive home.
The letters on the screen are not as clearly defined as on a printed page. Take a magnifying glass to your monitor and you"ll see the letters, each made up of pinpoint sources of light, have no sharp edges. But those liquid crystal display (LCD) monitors, like those found on laptop computers, are easier on the eyes because they display sharper images.
And studies have shown that when people are working on a computer, their rate of blinking goes down by two-thirds, which can result in dry, stinging eyes. This is especially a problem for contact-lens wearers.

______ used to be the biggest physical problem that computer users had.
A．Dry eyes
B．Blurred vision
C．Headache
D．Wrist pain
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