A Both of them turn understanding a new language into something tough and confusing.
B It is not relevant to learn about idioms and proverbs since they are not normative language.
C No matter if you know them or not, you are going to use idioms and phrasal verbs, eventually.
D It is important to be aware of them since idioms and proverbs increase our range of vocabulary and comprehensiveness, also enhancing our social skills.