In pyrosequencing, also known as "sequencing by synthesis," why do inaccuracies occur when the same base occurs greater than 6 times in a sequence?

A) Past a certain level of light intensity, it is more difficult to determine increases in light intensity
B) The reaction wells cannot be flushed fast enough to accurately measure the signal
C) Luciferin substrate is present in limiting amounts
D) Not all pyrophosphate generated will be converted to ATP