Norma receives $$0\mathrm{.}50$ per hour for each bag of candy she boxes and ships per hour. If she packages over $20$ bags per hour her rate increases to $$0\mathrm{.}60$ per hour per bag. This is an example of a:

Select one:

a$.$ merit pay increase.

b$.$ differential piecework plan.

c$.$ straight piecework plan.

d$.$ standard hour plan.

e$.$ straight commission plan.