The Purpose of the Photodetector in Immunologic Methods on Coagulation

The purpose of the photodetector in immunologic methods on coagulation analyzers is to:

A) Measure an increase in absorbance caused by the interaction of VWF with its monoclonal antibody
B) Measure an increase in transmittance caused by the interaction of D-dimer fragments in the patient's plasma with its monoclonal antibody
C) Promote the ball's oscillation movement through the electromagnetic sensors
D) Measure the amount of cleavage of p-nitroaniline