题目内容
—Is there any vaccine(疫苗) against H1N1 flu that can be used on humans?
—No, but scientists are working to develop ________ .
A. that B. it C. one D. so
C
练习册系列答案
相关题目
题目内容
—Is there any vaccine(疫苗) against H1N1 flu that can be used on humans?
—No, but scientists are working to develop ________ .
A. that B. it C. one D. so
C