A.a legend handed down from the past
B.a matter that is hidden or secret
C.a question difficult to answer
D.a problem not yet solved
[单选题]How men first learnt to invent words is unknown; in other words, the origin
[单选题]How men first learnt to invent words is unknown; in other words, the origin
[单选题]How men first learnt to invent words is unknown; in other words, the origin
[单选题]How men first learnt to invent words is unknown; in other words, the origin
[单选题]How men first learnt to invent words is unknown; in other words, the origin
[单选题]How men first learnt to invent words is unknown; in other words, the origin
[单选题]How men first learnt to invent words is unknown; in other words, the origin
[单选题]How men first learnt to invent words is unknown; in other words, the origin
[单选题]How men first learnt to invent words is unknown; in other words, the origin
[单选题]How men first learnt to invent words is unknown; in other words, the origin