题干
Scientists say it may be a few years ______ it is possible to test the new medicine on patients.
A：because
B：after
C：since
D：before
答案(点此获取答案解析）
D
Scientists say it may be a few years ______ it is possible to test the new medicine on patients.
A：because
B：after
C：since
D：before
和
都外切,则动圆圆心的轨迹为（ ）
