Sonia likes ice skates and also likes going skating. Her utility function for pairs of ice skates, S, and the number of times she goes skating per month, T, is U(S,T) = 3ST, so MUS = 3T and MUT = 3S. What is Sonia's marginal utility with respect to ice skates (dS)?

a) 3T
b) 3/T
c) 3
d) 3S