问题
选择题
| Scientists say it may be five or six years _____ it is possible to test this medicine on human patients. |
| A. since B. after C. before D. when |
答案
答案：C
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| Scientists say it may be five or six years _____ it is possible to test this medicine on human patients. |
| A. since B. after C. before D. when |
答案：C