What was discovered by accident? The answer is penicillin (青霉素). It kills germs. By killing germs, it saves lives. Suppose you are sick. You go to a doctor. She examines you. She says you have a “staph” infection (葡萄球菌感染). She gives you some medicine. You take it. The medicine knocks out the staph. Soon you are well. Before penicillin, this would not happen. Staph was almost sure death.

　　Everyone wanted a medicine. Laboratories worked day and night. They grew the staph in small dishes. Then they tried to kill it. Nothing worked.

　　The laboratory dishes had covers on them. They kept things from falling into dishes. Molds (霉菌) were a big worry. They are always in the air. You can’t see them. They’re too small. There are thousands of different molds. Molds can make an experiment (实验) fail. That’s why dishes are covered.

　　Dr. Alexander Fleming was working to kill the staph germ. He worked for years. One day he took a cover off a dish. He looked inside. There was a thick growth of staph germ. There was also some mold. Then he saw something strange. Where the mold was, there was no growth of staph. This is what Fleming probably thought. “By accident, I found a mold to kill the dreaded staph.”

　　This is how penicillin was found. But here’s the real miracle (奇迹). There are thousands of kinds of molds. But only one kind can kill staph. The mold must have fallen into the dish a few days before. The cover was probably off only a few seconds (秒). In those few seconds the right mold fell into the right dish. Another man might have thrown the dish away. But Fleming was very careful and smart. He understood what the mold did. How lucky the humans were!

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　　21. During the experiments to find a medicine for staph infections, ________.

　　A. the dishes were not covered

　　B. staph and mold were put together

　　C. mold was kept in small dishes

　　D. staph was grown in small dishes

　　22. The word “ dreaded” means “_______”.

　　A. famous    B. scary

　　C. health    D. endangered

　　23. The last paragraph suggests that _______.

　　A. accidents happen to everyone

　　B. the careful person does not have accidents

　　C. the careless person will miss valuable chances

　　D. luck is enough when doing scientific experiments

　　24. Most of this passage is written by ______.

　　A. telling an unusual story    B. showing the results of experiments

　　C. offering some reasons    D. giving some advice

　　25. This passage is mainly about ______.

　　A. medicine for a staph infection    B. useful accidents

　　C. Dr. Fleming’s research    D. discovering penicillin