Passage Three
In order for a chemical to be considered a drug, it must have the capacity to affect how the body works. No substance that has the power to do this is completely safe. [33]Drugs are approved only after tests have demonstrated that they are relatively safe when used as directed and when their benefits outweigh their risks. Thus some very dangerous drugs are approved because they are necessary to treat serious illnesses. Many people suffer ill-effects from drugs called side-effects even though they take the drugs exactly as directed. [34]The human population contains a great variety of genetic variation. But drugs are tested on just a few thousand people. When a particular drug is taken by millions, some people may not respond in a predictable way even though the drug has been tested. [35]A patient may also acquire a tolerance for a certain drug, which means the patient has to take ever-larger doses to produce the desired effects. Tolerance may lead to heavy situation, in which a person becomes so dependent on the drug that he or she becomes addicted to it. Addiction causes severe psychological and physical disturbances when a drug is taken away. Finally, drugs often have unwanted side-effects. They usually cause only minor discomfort, such as a skin rash, headache or sleepiness. Certain drugs, however, can produce serious adversary actions.

Questions 33 to 35 are based on the passage you have just heard.
33. Under what circumstances are drugs approved?
正确答案：D
解析：短文听力常常是“听到什么选什么”。录音提到，药物只能在经过安全测试后，并在指导下使用。故答案为B。
34. Why do many people suffer side-effects from a drug even though they take it as directed?
正确答案：C
解析：录音提到，人们按规定服药后会出现副作用，接着解释，人类包含了很多遗传变异，但是药物测试只在部分人中间进行。由此可以概括出C。
35. What will happen when patients acquire a tolerance for a certain drug?
正确答案：A
解析：录音提到，病人也可能会对某种药产生抗药性，这就意味着病人要加大剂量，以达到预期的药效。由此可知答案为A。