单选题：Questions are based on the following passage．Every fall，like

Questions are based on the following passage．Every fall，like clockwork，Linda Krentz of Beaverton，Oregon，felt her brain go on strike．“I just couldn’t get going in the morning,”she says．“I’d get depressed and gain 10 pounds every winter and lose them again in the spring．Then she read about seasonal affective disorder，a form of depression that occurs in fall and winter，and she saw the light-literally．Every morning now she turns on a specially constructed light box for half an hour and sits in front of it to trick her brain into thinking it’s still enjoying those long summer days．It seems to work．Krentz is not alone．Scientists estimate that 10 million Americans suffer from seasonal depression and 25 million more develop milder versions．But there’s never been definitive proof that treatment with very bright lights makes a difference．After all，it’s hard to do a double-blind test when the subjects can see for themselves whether or not the light is on．That’s why nobody has ever separated the real effects of light therapy from placebo （安慰剂）effects．Until now．In three separate studies published last month，researchers report not only that light therapy works better than a placebo but that treatment is usually more effective in the early morning than in the evening．In two of the groups，the placebo problem was resolved by telling patients they were comparing light boxes to a new anti-depressant device that emits negatively charged ions （离子）．The third used the timing of light therapy as the control．Why does light therapy work？ No one really knows．“Our research suggests it has something to do with shifting the body’s internal clock,”says psychiatrist Dr．Lewey．The body is programmed to start the day with sunrise，he explains，and this gets later as the days get shorter．But why such subtle shifts make some people depressed and not others is a mystery．That hasn’t stopped thousands of winter depressives from trying to heal themselves．Light boxes for that purpose are available without a doctor’s prescription．That bothers psychologist Michael Terman of Columbia University． He is worried that the boxes may be tried by patients who suffer from mental illness that can’t be treated with light． Terman has developed a questionnaire to help determine whether expert care is needed．
In any event，you should choose a reputable manufacturer． Whatever product you use should emit only visible light，because ultraviolet light damages the eyes． If you are photosensitive （对光感的），you may develop a rash． Otherwise，the main drawback is having to sit in front of the light for 30 to 60 minutes in the morning． That’s an inconvenience many winter depressives can live with．What is the probable cause of Krentz’s problem？ A．An unexpected gain in body weigh
B．Unexplained impairment of her nervous syste
C．Weakening of her eyesight with the setting in of winte
D．Poor adjustment of her body clock to seasonal change