Most Clinical Laboratory Assays Measure Concentration of Analyte in the Blood

Most clinical laboratory assays measure concentration of analyte in the blood stream.Why do most enzyme analyses measure enzyme activity instead of enzyme concentration?

A) The assays utilized to measure enzymes involve equilibrium reactions where the reaction can proceed in both directions during the assay.
B) The enzyme being analyzed may be present in such a large concentration that it is difficult to maintain linearity, causing activity measurements to be more suitable.
C) Enzymes are not consumed during the reactions in which they are involved, requiring analysis by activity instead of concentration.
D) The products formed by enzymatic reactions are not sufficiently chromophoric to be measured by traditional spectrophotometric techniques.