A Every healthy human being has the right of speaking his/her own language, with no need of investigation.
B As language occurs to be natural to those who listen and speak correctly - according to normative grammar rules - there is no need to investigate it.
C Once we bear in mind that knowing a language is something innate, we, as people, truly believe everything in language is naturally explicable.
D Because people in general only consider the perspective of language as users, they do not realize how complex languages can really be.