The government must take measures to prevent the river __________.
Low-cost books in paper covers make ______ some of the world’s best literature to readers.
He must be helping the old man to water the flowers, ______?
—Can I pay the bill by check?—Sorry, Madam, but it is our managing rule that payment ______ be made in cash.
The magazine you lent me ______ among the 10 most popular ones in our school.
Professor Smith, along with his assistants, ______ on the project day and night to meet the deadline.