某光源能够发出两种波长的光，波长分别是500nm和580nm，对应的明视觉下光谱光视效率分别为0.323和0.995，对应的暗视觉下光谱光视效率分别为0.998和0.130，其单色辐射通量分别为5W和2W，向四周均匀发射光通量。可利用第19题的计算结果，求发光强度。
A、68cd
B、196cd
C、285cd
D、784cd

单项选择题8. Which event happened before the photo was taken? A、Watergate B、Vietnam conflict C、911 terrorist attack D、Surprise attack on Pearl Harbor

单项选择题Which of the following does not help when you try to identify the main idea of an essay? A、The type of essay. B、Notions which seem to be repeated throughout. C、The selection of sentence structure. D、None