They eat less sugar and that is ________ they have ________ white teeth.
A.because, healthyB.why, healthC.because, healthD.why, healthy在线课程D
解析：
考查表语从句。此题考查了两个知识点①that is why…意为“……的缘故”；
that is because…意为“那是因为……”②health是名词healthy是形容词。从题干所给的意义分析第一个空格只能用why，第二只能用作宾语的形容词healthy。所以正确答案为D。