The answer lies in two associated differences among the species, in their digestive systems and body sizes.

答案在于两个相关的物种之间的差异，它们的消化系统和体型。

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Most medical students spend their first two years mainly in classroom study. They learn about the body and all its systems. They also begin studying how to recognize and treat disease.

多数医学院校学生前两年都在教室里学习。他们学习人体及其各个系统的知识。也学习如何诊断和治疗疾病。

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Steering and suspension systems are two key systems in affecting body attitude and riding safety in automotive chasis systems.

转向与悬架系统是汽车底盘系统中影响车身姿态和行驶安全性的两大关键系统。

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In fact, the two examples which buck the trend - the two body systems which do clearly deteriorate in middle age - make my point for me.

BBC: Viewpoint: Just what is middle age?