种母猪的繁殖性状的遗传力估计值较低，选种时一般以家系选择为好。()
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How thd NASAs investigators prove the reason of thsaster？
A. By studying the video tape.
By analyzing the data concerned.
C. By anatomizing seven astronauts" body.
D. By making a simulated experiment.
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。()
What is the talk mainly about？
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.
According to the conversation, why wont DME be on the market soon？
A. It hasn"t been adequately tested.
B. It"s expensive.
C. It damages car engines.
D. It"s dangerous to transport.
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