根据短文内容和所给汉语提示,写出短文空白处各单词的正确形式.每空限填一词.
For thousands of years,people have used plants to make medicines.They used different parts of the plants-the roots,the leaves,the flowers,and the bark (树皮).Today,doctors have rediscovered more medicinal values (药用价值) of some plants.Let's look at some examples.
　　Foxglove is a common plant.People have used it to make the heart slow down.Recently scientists have developed another drug from it.This new drug helps prevent other heart problems.
　　In some areas,malaria has been a problem.Scientists have learned that the bark of a South American tree,the cinchona,can be made into a drug to prevent malaria.For a long time,the Chinese have known that a special plant,wormwood,can also fight malaria.Scientists have been working with it to develop new drugs against malaria.
　　In Germany,some scientists studied garlic (大蒜) for four years.They found that it helps prevent the build-up of plaque (血小板) because too much plaque is bad for health.This very common plant continues to be studied.
　　As we know,there are about 250,000 kinds of flowering plants in the world.Scientists have only studied little more than one percent of plants for their medicinal value.Maybe medicines for AIDS and cancer will be made from the other 99 percent.
　　52.What can foxglove be used for?
　　A.Studying plants.B.Building up the plaque.
　　C.Treating AIDS and cancer.D.Preventing heart problems.
　　53.What does the underlined word "malaria” in Paragraph 3 refer to (指的是)?
　　A.A drug.B.A scientist C.An illness.D.An animal.
　　54.How many plants are mentioned in the passage?
　　A.3.B.4.C.5.D.6
　　55.Which of the following is the best title for the passage?
　　A.Past and future B.Plants and medicine~
　　C.Health and illness D.Scientists and doctors