A firm is hiring 9 workers at $8.00 per hour. In order to hire an additional (10th) worker it must raise the wage it pays (to all workers) to $8.50 per hour. The marginal factor cost to the firm of hiring the 10th worker is
a) $13.00
b) $8.00
c) $64.50
d) $8.25
e) $8.50