Предложение поставить во времена Present, Past, Future Indefinite; Present, Past, Future Continuous; Present, Past, Future Perfect; Present Prefect Continuous. Предложение: The practice of controlling or preventing a monopoly varies from country to country.

present1. The practice of controlling or preventing a monopoly varies from country to country2. is varying 3. has varied4. has been varyingpast1. varied2.  was varying 3. had varied4. had been varyingfuture1. will vary2. will be varying 3. will have varied4. will have been varying