矿物质元素对羊生长有着重要影响，缺少时常会带来相应的疾病，若土壤中缺铜，羊易患“白肌病”;土

伸度(名词解释)

绵羊的性成熟期一般在()，初次配种年龄是()左右。

根据生产方向分类法将山羊品种分为哪几种类型的品种？

什么叫杂交？杂种优势？

Why are boys more at risk, especially the overweight boys？

A．Boys like vigorous sports, so their heart bear more burden.

B．Boys don"t like go to hospital to receive ultrasound treatment.

C．Their hearts are usually too big to pump blood very well.

D．Their hearts are over worked to support their bodies if they are overweight.

请帮忙给出正确答案和分析，谢谢！