You are designing an x-ray imaging system to image two different types of tissue whose linear attenuation coefficients are shown in the graph below. This imaging system should allow the clinician to most easily tell the two tissue types apart. You have two options for an x-ray tube: one emits primarily bremsstrahlung with Emax=100 keV and the other emits primarily characteristic radiation at 20 keV. Which x-ray tube do you choose?