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.

Present
1. The practice of controlling or preventing a monopoly varies from country to country
2. is varying
3. has varied
4. has been varying

past
1. varied
2.  was varying
3. had varied
4. had been varying

future
1. will vary
2. will be varying
3. will have varied
4. will have been varying