To make a shelf to display her teapot collection, Amina bought wood and nails. Each yard of wood costs $27 and each box of nails cost $4. She bought 6 fewer boxes of nails than yards of wood. Use this information for number 21 and 22.

21. Let x represent the number of yards of wood that Amina bought. Which expression could Amina use to represent the number of boxes of nails that she bought?
A. x
B. 4
C. x - 6
D. x + 6

22. Amina's total cost was $193. How many boxes of nails did she buy?