对一些具备关键性影响且外部性较强的基础研究活动，应主要由（）

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问题单项选择题

对一些具备关键性影响且外部性较强的基础研究活动，应主要由（）来承担。

A.国有研发机构

B.混合所有制研究机构

C.军队研究机构

D.民间科技企业

答案

参考答案：A

选择题

_____ three times in a row, the boxer decided to give up fighting. [ ]

A. Having defeated

B. To have defeated

C. Having been defeated

D. To have been defeated

填空题

如图所示，A、B为两质量相等的小球，由细线相连，再用轻质弹簧悬挂起来，将A、B间细线烧断的瞬时两球的加速度大小分别为__________、__________，方向分别为__________、__________。