$4.$ A student is monitoring a reaction involving compounds C and D using TLC$.$ Compound C has an Rf value of $0\mathrm{.}43$; compound D has an Rf of $0\mathrm{.}40.$ a$.$ Draw a picture of the TLC plate at the beginning of the reaction. b$.$ Draw a picture of the TLC plate after $50\%$ completion $(50\%$ of the C molecules have been converted to D molecules$).$ Draw a picture of the TLC plate at the end of the reaction $($all of the C molecules have been converted to D molecules$).$ d$.$ Why is it important to use a co$-$spot?