放射性活度是What's radioactivity?
A: 单位体积内放射性核素的数目Number of radionuclides per unit volume
B: 单位时间内发生核衰变的次数Number of radionuclide decay per unit time
C: 单位时间内核衰变释放的能量Energy released by radionuclide decay per unit time
D: 核素半衰期的2倍2 times the half-life of the radionuclide
E: 放射性核素半衰期Half-life of radionuclides