Suppose that you are given a $\$ 100$ budget at work that can be spent only on two items: staplers and pens. If staplers cost $\$ 10$ each and pens cost $\$ 2.50$ each, then the opportunity cost of purchasing one stapler is:
a. 10 pens.
b. 5 pens.
c. zero pens.
d. 4 pens.