A family with 2 adults and '2 children and with clothes washer and dishwasher should purchase a water heater with a capacity of ______ gallon.

A．a textbook intended for technicians
B．a consumer guide
C．a book on the history of electric appliances
D．an operation manual for a type of water heater

A．the history of Jazz
B．the themes of pop music
C．the varieties of world music
D．the importance of music in everyday life