50. The reason why a great barrier has been built across the Thames at Woolwich is that_________.

A. London is very slowly sinking into its foundations

B. the level of the river Thames is slowly rising

C. extra-high tides could flood a large part of London

D. all of the above.

C

“That’s funny! These fellows in the middle of the plate have died.” Dr Alexander Fleming was talking to another doctor in a laboratory in London. He had been studying some germs(细菌)that he was growing on a plate. They were very dangerous germs because they caused different kinds of illnesses that could kill people. Dr Fleming found that a mould(霉菌)had floated in through the window landing on the plate. It had killed some of the germs it had touched.

“This certainly looks promising.” Fleming said. “We must grow some of this mould to see if it will kill other germs.”

He named the strange mould “penicillin”. It proved to be a killer of many germs. Fifty mice were given deadly germs and then half of them were injected(注射)with penicillin. The twenty-five untreated mice died, but twenty-four of those lived that had been treated with penicillin. Dr Fleming wrote a report about what he had found out. Hardly anybody took any notice of it.

In 1938 Dr Howard Florey, an Australian working in London, read Dr Fleming’s report and was very interested. He found that penicillin was effective in treating blood poisoning in human beings.

When World War II broke out, it was not possible to make enough penicillin in England. Dr Florey went to America where he helped to have enormous amounts of this wonderful drug made. It saved the lives of thousands of soldiers, sailors and airmen who would have died from their wounds if the hospitals had not had penicillin.

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