答案:D
选项A:两者都,其后谓语动词应为复数,即应为BothBetty's parents and Betty like Chinese culture,故排除A;
选项B:要么…要么…,符合"就近原则",但与题干That's why they are learning Chinese this year(那就是为什么他们今年正在学习汉语)题意不符;
选项C:既不…也不,也符合"就近原则",也与题干That's why they are learning Chinese this year(那就是为什么他们今年正在学习汉语)题意不符;
选项D:不但…而且…,该短语有"就近原则"即谓语动词跟随最近的名词而变化,此题Betty决定谓语动词为likes,符合题意.故选D.