Consider a Model of Inter-Temporal Choice, with Utility Function $u\left(c_{1}, c_{2}\right)$

Multiple Choice

Consider a model of inter-temporal choice, with utility function $u\left(c_{1}, c_{2}\right)$ . If $c_{1}$ and $c_{2}$ are substitutes then which of the following must be true?

A) $\frac{\partial u\left(c_{1}, c_{2}\right) }{\partial c_{1}}>0$ .
B) $\frac{\partial u\left(c_{1}, c_{2}\right) }{\partial c_{2}}<0$
C) $\frac{\partial^{2} u\left(c_{1}, c_{2}\right) }{\partial c_{1} \partial c_{2}}>0$ .
D) $\frac{\partial^{2} u\left(c_{1}, c_{2}\right) }{\partial c_{1} \partial c_{2}}>0$