某小儿患佝偻病，发烧时就会抽搐，根据所学知识，建议他平时要补充（   ）

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    Mr. Black is American, but he is in Canada now. He is a tall man with funny glasses. He works in a big city but lives in a town far away. He lives on the twentieth floor of a building. Every morning he gets up early. Then he gets into the lift and it takes him down to the first floor. He runs to the bus stop and catches the early bus. It usually takes him about two hours to get to the factory by bus. Then he gets off the bus and walks into the workshop(车间) quickly. He starts his work at eight in the morning.

如图，在直角坐标系中，矩形OABC的顶点A在x轴上，顶点C在y轴上，B（4，3），连接OB，将△OAB沿直线OB翻折，得△ODB,OD与BC相交于点E,若双曲线 $y=\frac{k}{x}(x>0)$ 经过点E,则k=____。

“封建亲戚，以藩屏周”反映了西周时期实行的政治制度是（    ）

下图表示在某生态系统中，a、b、c三个种群数量变化的相互关系，下列描述正确的是(       )