When the bacterium Serratia marcescens is grown on a sterile medium in a petri dish at 30° C, the bacterial colonies are cream colored. When this same bacterium is cultured under identical conditions, except at a temperature of 25° C, the colonies are brick red. This difference in color is most likely due to the a. type of nutrients in the culture medium
b. sterilization of the culture medium
c. effect of temperature on the expression of the gene for color
d. effect of colony size on the synthesis of color pigments