you are given 5 substances: backing material, pzt, matching layer, gel, and skin whose impedances have been measured. unfortunately, the labels identifying each have fallen off. there are five labels, indicating impedances of 0.7 mrayls, 1.0 mrayls, 1.8 mrayls, 1.5 mrayls, 2.0 mrayls. if the backing material is 1.8 mrayls, what are the impedances of the soft tissue, matching layer, gel, and pzt. why?