21.A.根据 Du Chinese这个部分的表述: You can easily turn the Pinyin on and off and switch Chinese characters between Simplified and Traditional.可知选项A正确。22.D.根据 Skritter这个部分的表述: Another great feature Skritter offers is the ability to track your progress. They keep track of how many characters are introduced. how many are practiced, and how many are memorized可知选项D正确。而选项A表达不符合原文“muscle memory这个表述;选项B与原 vocabulary is taken straight from textbooks的表述不符;选项C是无中生有。23.C.根据文章中推荐的Apps,可知能教笔画顺序的是 Skritter和 Pleco,而教 components的是 Zizzle,因此是三种Apps.