A．They are sold at lower prices than in other shops.
B．They are very-popular with the local residents.
C．They are delivered free of charge.
D．They are in great demand.

A．To follow the custom of the local shopkeepers.
B．To attract more customers in the neighborhood.
C．To avoid being put out of business in competition.
D．To maintain friendly relationships with other shops.

A．They can be used to deliver messages in times of emergency.
B．They deliver pollutants from the ocean to their nesting sites.
C．They carry plant seeds and spread them to faraway places.
D．They are on the verge of extinction because of pollution.