Chapter 2, Problem 15PS

Which of the following is not an integer constant in C?

$\begin{array}{l}\text{a}\text{. }-320\\ \text{b}\text{. }+45\\ \text{c}\text{. }-31.80\\ \text{d}\text{. }1456\\ \text{e}\text{. }2,456\end{array}$