Make the line down in parbox

Question

Why do I get a line in the middle?

\documentclass{article}

\begin{document}
%%\parbox[alignment][height][inner-alignment]{width}{text}
The answer is:\parbox[c][][b]{2cm}{\hrulefill}.\par
\end{document}  

Answer 1

A \parbox is certainly not the right tool for this job.

If you want a 2cm wide rule sitting at mid height of lowercase letters, you can do

\documentclass{article}

\newcommand{\middlerule}[2][0.4pt]{%
  \raisebox{\dimexpr0.5ex-(#1)/2}{\rule{#2}{#1}}%
}

\begin{document}

The answer is:~\middlerule{2cm}

\end{document}

Or, with the \vrule primitive,

\documentclass{article}

\newcommand{\middlerule}[2][0.4pt]{%
  \vrule
    height \dimexpr0.5ex+(#1)/2\relax
    depth \dimexpr(#1)/2-0.5ex\relax
    width #2\relax
}

\begin{document}

The answer is:~\middlerule{2cm}

\end{document}

With either version, the input

The answer is:~\middlerule[1pt]{2cm}

would produce a 1pt thick rule.