单选题
When scientists want to test a new drug，they usually divide a large number of people 16 two groups. One group takes the medicine and the other takes a substance 17 as a placebo (安慰剂). It may contain nothing more than sugar. The people do not know which pill they are taking, the active one or the inactive one. In this kind of experiment, the medicine must perform 18 than the placebo to prove it is effective.
Yet, people who take a placebo sometimes experience improvements in their health. This is known as "the placebo effect" —the effect of something 19 is not supposed to have any effect.
Some doctors even use the placebo effect in their treatments. They might tell patients that a new drug will stop their pain. The patient does not know that the pills are 20 . The patient 21 the pills and later tells the doctor that the pain is gone.
Now research in Sweden suggests that placebo treatments can also 22 the emotional effects of unpleasant experiences. The effects in the brain were similar to those seen when placebos have been used to ease pain. The researchers say that in both cases expectations of improvement are a major 23 on the effectiveness of placebos. The new study involved a group of people who looked at unpleasant pictures, 24 images of dead bodies. Predrag Petrovic of the Karolinska Institute in Stockholm led the study. The findings appeared in the publication Neuron (神经元).
An influential study on placebos appeared in 1955. It said treatment with a placebo 25 patients feel better 35% of the time. But in 2001, Danish researchers reported that they had examined more than 100 studies. They found 26 evidence of healing as a result of placebos.
Some researchers think a good relationship between a doctor and patient can increase the effectiveness of real medicines. In any case, some medical researchers are against the use of 27 . They think it is 28 to give some people inactive substances when testing new medicines. They say it would be better to 29 new drugs with existing drugs. That way, a study would show 30 the new drug is more effective.