What Is the Purpose of the Photodetector on the Diagnostica

What is the purpose of the photodetector on the Diagnostica Stago STA Compact?

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