How might a trait travel over great distances?

Below is the complete question. 

Many different traits make up a culture. During an exchange known as cultural diffusion, some of those traits are spread from one culture to another. How do you think cultural diffusion works? How might a trait travel over great distances? What cultural traits do you think are the most likely to be passed from culture to culture?

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The first trait is that the region is rural in general character.Rural suggests open country,farming and contrasts to city life(+)

The answer is b im not sure 

Answer primary waves;