英语阅读理解."Mind over matter" is an English saying meaning that

英语阅读理解.
"Mind over matter" is an English saying meaning that we can control our bodies with our minds.Now scientists are finding increasing evidence that our mind can have a great effect on our physical health.Since our mind influences how we feel ,it's not possible to say if someone is ill just because they feel ill.
The influence of the mind on how well we feel is the basis of what is known as the placebo effect.A placebo (which is Latin for "I shall please") is a harmless,inactive substance.Patients are sometimes given placebo pills and begin to feel better because they believe that they have been given real drugs and expect to respond to them.The placebo effect is very strong:research on pain suggests that up to 30% of the effect of most painkillers is a placebo effect.
So if it is so powerful,why don't doctors use it today?
According to Dr Persaud,they really do."All these things like wearing a white coat,going to a hospital,and being scanned have a very strong placebo effect,"he says.
From the passage it can be seen that there will be no placebo effect if a patient_____.
A.is dressed in white
B.has a physical exam
C.takes sugar pills
D.sees a doctor
为什么选A?根据什么推导出来的?