计算CT值的公式是根据The formula of CT value is based on
选项：

A:水的质量衰减系数Mass attenuation coefficient of water;
B:水的线性衰减系数Linear attenuation coefficient of water;
C:水的电子密度Electron density of water;
D:水的密度Water density;
E:空气的质量衰减系数Mass attenuation coefficient of air