How to use Courier in exam class

Question

I am writing an exam and I want to use the Courier font, however the produced document has no Courier. Can someone help me in producing the document I need.

\tracinggroups=1
\documentclass[12pt,addpoints,answers]{exam}
\usepackage{m-ch-en}
\usepackage{chemfig}
\usepackage{courier}
\usepackage{here}

\begin{document}
\begin{questions}
\question  Eyepiece graticules and stage micrometers are used to measure cells.\\
Which is the correct reason why an eyepiece graticule is calibrated?
\begin{choices}
\choice An eyepiece graticule can be used to make measurements.
\choice An eyepiece graticule is magnified by the objective lens.
\choice An eyepiece graticule magnifies the specimen.
\choice An eyepiece graticule makes comparisons.
\end{choices}
\question When mucus is secreted from a goblet cell in the trachea, these events take place.
\begin{enumerate}
\item addition of carbohydrate to protein
\item  fusion of the vesicle with the plasma membrane
\item  secretion of a glycoprotein
\item  separation of a vesicle from the Golgi apparatus
What is the sequence in which these events take place?
\end{enumerate}
What is the sequence in which these events take place?
\begin{choices}
\choice $1 \rightarrow 4 \rightarrow 2 \rightarrow 3$
\choice $1 \rightarrow 4 \rightarrow 3 \rightarrow 2$
\choice $4 \rightarrow 1 \rightarrow 2 \rightarrow 3$
\choice $4 \rightarrow 1 \rightarrow 3 \rightarrow 2$
\end{choices}
\question What is a function of the smooth endoplasmic reticulum?\\
\begin{oneparchoices}
\choice  antibody synthesis
\choice enzyme synthesis
\choice protein synthesis
\choice steroid synthesis
\end{oneparchoices}
\question Which of the following can decrease the fluidity of the cell surface membrane?
\begin{itemize}
    \item  double bonds between carbon atoms in the fatty acid chains
    \item  cholesterol
    \item  fatty acids having longer chains
\end{itemize}
\begin{choices}
    \choice  1, 2 and 3
    \choice 1 and 2 only
    \choice 2 and 3 only
    \choice 3 only
\end{choices}
\end{questions}
\end{document}