Consider a household utility function, U(C), where c is household consumption. We typically assume that household utility is "concave" in consumption. Mathematically, this means that
a. ((∂²U(C)) / (∂C²)) >0
b. ((∂U(C)) / (∂C)) >0
c. ((∂²U(C)) / (∂C²)) =0
d. ((∂²U(C)) / (∂C²)) <0