Answer:
C> B> A
Explanation:
Order of increasing wavelength
ultraviolet< visible < infrared
The wavelength being blocked depends on the size of the dust grain. Larger the grain, larger wavelength it be able to block.
Region A obscures wavelengths of light shorter than ultraviolet. Region B obscures wavelengths of light shorter than visible light and Region C obscures wavelengths of light shorter than infrared light. Region C must have largest grains as it is able to block longer wavelengths. Thus, the rank of dust grains based on their size is C > B > A