关于玉米的密植育种和生产还有很多不确定因素。
There were many uncertain factors to high density maize breeding and production.
也正因为这个外部性，人力资本的价值分析也带来了很多不确定因素。
Also because of it, analyzing the value of human capital becomes very difficult and uncertain.
我深知这一历史时刻的到来恰逢对很多美国人来说有很多不确定因素之时。
Iknow this historic moment comes at a time of great uncertainty for many Americans.
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