酶活力是酶催化速度的量度指标，酶的比活力是酶纯度的量度指标，酶（）是酶催化效率的量度指标
参考答案：转换数
______ you go across the road, you must be careful. [ ]
A. Since
B. After
C. When
D. Before
何谓骨折？骨折有何特殊体征？
