What is the function of the light pipe on each print cartridge?

A.
verifies installation of print cartridges in the correct slot

B.
works with the detection sensor to detect print cartridge toner level

C.
notifies the DC controller of print cartridge errors

D.
controls the operation of applying toner to the development roller

Explanation:

Reference:
http://www.fixyourownprinter.com/forums/printer/53450