Diet varies _______ different countries in the world.
A. between B. among
请问怎么样选择？同时请老师具体谈谈它们的使用方法。
你的句子填 between, among 都不算错。
vary 后接介词 between 或 among 的具体使用方法大致如下：
1. 假如vary后面接的是 A and B, A, B and C 之类含有连词 and 的词组，原则上应用 between。如：
Class numbers vary between 25 and 30. 班级的数目从 25 到 30 不等。
Her feelings varied between joy and fear as she watched him fight. 当她现看他拳击的时候，时而喜悦，时而担忧。
The origin of the calendar is inaccurate to begin with, and our “year” is not strictly astronomically accurate but varies between 365 and 366 days. 历法的来源本来就不精确，何况大家的“一年”在天文学上又不是严格算准而是介乎365与366天之间。
2. 假如vary后面接的是一个复数名词，按理说应该用 among，但事实上也常常用介词 between（表示“每两者之间”）。如：
Salary scales vary between states / from state to state / according to state / with each state. 薪资高低各州 / 一个州与另一州 / 视所在的州 / 每个州不同。（《《剑桥国际英语词典》》）
Services offered vary among the main high street banks. 各大商业银行提供的服务不尽相同。（《牛津英语搭配词典》）
