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Exercises. 1. Complete the sentences with the verb in the correct form: Present Simple, Present Perfect, Past Simple Tense
1. Complete the sentences with the verb in the correct form: Present Simple, Present Perfect, Past Simple Tense.
Virus diseases …..( plague) mankind for thousands of years, but, until recently, no medical treatment for them …..( be) available. Although the use of vaccines almost …..(eliminate) several major viral diseases, such as polio and smallpox, medical research …..( provide) no means for treating virus diseases already established in the body. No drugs …..( be) made available for combating viruses even though drugs such as penicillin have been used so effectively against bacterial infections.
Ever since 1957, when interferon was discovered, there …..( be) the hope that this natural antiviral substance produced by cells might provide an effective treatment for a broad range of virus diseases that are not subject to control by antibiotics. Not for the first time firm evidence has been presented that an animal's body can be stimulated to produce enough interferon to cure an acute infection. An eye disease in rabbits was treated by administering an agent that …..(raise) the animals' interferon level and …..(clear) up the infection.
Interferon …(be) a protein, produced by cells under virus attack, that in turn apparently … (give) rise to a factor that limits the spread of the virus. The interferon system has been demonstrated in the laboratory and in human beings, but the body's own system often …..( not produce) enough interferon to overcome an established infection. Treatment with prepared interferon can prevent many viral infections and …..(have) some protective effect when it is given after inoculation with a virus and before the onset of disease, but it is hard to accumulate enough prepared interferon to treat an established infection.
Many workers in virology, biochemistry, and molecular biology …..(study) interferon, and out of this research …..(come) the rationale for an entirely new approach to the treatment of viral disease-induction of the synthesis of interferon in the body in order to stimulate the cell's natural defence mechanism.
2. Complete the sentences with the verb in the correct form: Present Simple, Present Perfect Continuous, Present Perfect Tense.
1) Men …..( dig) up fossils for centuries, but it is only in the last two hundred years that they generally ….. (recognize) as the remains of living things.
2) If disturbed, earwigs run away very quickly, but if they …..( hide) in a lettuce, they cling on tenaciously and they manage to keep afloat in water for quite a while.
3) A smoke-cloud darkened the sky and the earth …..( tumble) for days.
4) The seals …..( breathe) through tiny cracks in the ice. These conclusions are substantiated by Arctic explorers and research workers who …..(study) the northern hunting grounds.
5) Mankind ….. (eat) corn for so many hundreds of years that today we ….. (think) of it as just another fine vegetable.