This Payoff Matrix Gives Potential Dollar Gain Values in Thousands

This payoff matrix gives potential dollar gain values in thousands for strategies S1, S2, S3, and S4 for Sam's Pizza and competitive strategies CA1, CA2, and CA3 for Pam's Pizza.What is the maximum regret value for S1? CA1 CA2 CA3
S1 13 14 7
S2 7 17 12
S3 31 29 4
S4 20 12 21

A) 18
B) 15
C) 14
D) 12