Buying local is a huge trend even in (city/cities) that have many branches . 是複數與否？為何？

> Buying local is a huge trend even in (city/cities) that have many branches .
1. Buying local is a huge trend even in city that have many branches (X)
2. Buying local is a huge trend even in cities that have many branches. (O)
= 在很多分部（ｂｒａｎｃｈｅｓ）的都市們之中，＂買土產＂是大流行中．

For the past several years, AmExp has been promoting "shop small", which is more realistic than "buy local", but turned out to be a failure. Local productions, barring certain very few industries like produces, still are much more expensive.

這句話重點是句子结構,關係子句that have many branches當形容詞修飾其先行詞cities，而動詞have其邏輯主詞是cities才可用複數。另外that亦可以which代替。若没有關係子句限制其範圍，則city與 cities均可使用。