A spectrophotometer measures turbidity, but this measurement can be used to estimate cell number if you first generate a standard curve that relates CFU per ml culture (on the y-axis) to optical density (on the x-axis) across a range of cell concentrations for your culture. This method generally works well, but there can be some problems. If you were using this standard curve approach, which of the following would cause you to mis-estimate the number of CFU per ml in a culture from a measurement of optical density.

a. the culture increases in density from one measurement to the next
b. the culture produces a dark brown pigment under certain conditions
c. the cells in the culture start clumping together
d. the culture decreases in density from one measurement to the next
e. the culture stops growing