单项选择题How man first learnt to invent words is unknown; in other words, the origin of language is a mystery. All we really know is that man, unlike animals, somehow invented certain sounds to express thoughts and feelings, actions and things, so that they cou The origin of language is ().
A.clearly explained in this passage
B.a matter that can not be or has not been explained or understood
C.a question that was difficult to answer
D.a problem already solved