The following units of a particular item were available for sale during the period: Beginning inventory 40 units at $20 First purchase 50 units at $21 Second purchase 50 units at $22 Third purchase 50 units at $23 What is the unit cost of the 35 units on hand at the end of the period as determined under the periodic inventory system by the FIFO costing method?

a. $20

b. $21

c. $22

d. $23