国家开放大学 / 综合英语（1）
正确率：100%
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
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
参考答案：
佳题速递：