繁育体系的好处是充分利用杂种优势，加快纯系的遗传进展。有利于控制种源。()

A. Overland transportation in the nineteenm century.
B. Historical aspects of mail delivery.
C. Vehicles currently in use by the postal service.
D. The invention of the railroad.

A. It hasn"t been adequately tested.
B. It"s expensive.
C. It damages car engines.
D. It"s dangerous to transport.

A. She was impressed by it.
B. It was a waste of money.
C. She was amazed that it had opened so soon.
D. She didn"t like it as much as the other wings.

A. History teachers are not patient.
B. Students are not hardworking.
C. The professors don"t make it attractive.
D. Students don"t think it important.