普朗克定律(Planck's distribution law)描述的是( ) A: 黑体发射力(Total, hemispherical blackbody emissive power) B: 黑体光谱半球发射力(Spectral, hemispherical blackbody emissive power) C: 黑体光谱定向发射力(Spectral, hemispherical blackbody emissive power) D: 光谱辐射强度(Spectral Intensity of the emitted radiation)