它们是农学院培育出的改良品种。

They are improved strains bred by the Agricultural College.

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其中16个为农家品种，12个为改良品种。

These varieties consisted of 16 land RACES and 12 im proved varieties.

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村民组织正在努力增加这种改良品种的供应。

Organizations of village farmers are working to expand the supplies of the improved seeds.

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You need hybrids, in order to improve the category of what you're doing.

你必须用杂交来改良品种

耶鲁公开课 - 古希腊历史简介课程节选