单项选择题
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
A．They are used to express feelings only. B．They can not be written down. C．They are simply sounds. D．They are mysterious.
A．they could agree upon certain signs B．they could write them down C．they could communicate with each other D．they could combine them