摘要： -Is there any vaccine against H1N1 flu that can be used on humans? -No, but scientists are working to develop . A. that B. it C. one D. so
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---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