根据汉语提示完成单词。

1. The man gave a lot of money to a medical _______ (研究)．

2. He _______(栓) his horse to a tree by the road just now.

3. People get _______ (紧张) when they go to work the first time.

4. Our teacher asks me to give a _______ (演讲) next Monday.

5. We hope your children grow happily and _______(健康)．

已知某企业生产铜丝，其直径X～N(32.5，1.1²)，现从该企业生产的一批铜丝中随机抽取6个样品，测得直径均值X＝31.13mm，假定现在生产的铜丝的标准差未变。

a＝0.05时，推断铜丝直径均值是（）。附：μ_0.95=1.645，t_0.95（5）=2.015，（5）=12.83，t_0.0975（5）=2.571，u_0.975=1.96

A．变小

B．未变小

C．不能确定

D．变大