Theoriginsofhumanspeechremainamystery,__________wehaveafairlyaccurateideaofwhenwritingbegan.
A.once B.if C.though D.since正确答案C
Theoriginsofhumanspeechremainamystery,__________wehaveafairlyaccurateideaofwhenwritingbegan.
A.once B.if C.though D.since正确答案C