The bee population has a decremental fashion over time. Elaborately state one potential extra variable that might be the culprit of the process packed with some tips of one correlation term: cause and effect.
a) Pesticide usage with a correlation to bee mortality rates
b) Decrease in floral diversity with a correlation to habitat loss
c) Climate change with a correlation to altered flowering patterns
d) Parasite infestation with a correlation to weakened immune systems