Writing clean long underscores

Question

Possible Duplicate:
Long underscore in LaTeX

I'm writing a document in which I encourage the reader to "fill in the blanks" in order to be actively engaged with the proof.

Is there any way to write a clean, long underscore? Currently I'm using \underline{\hspace*{2cm}}, but it doesn't look so great...

Here's a sample of my text:

\begin{thm}

$A \subset B$ if and only if $A \cap B = A$.

\end{thm}

\begin{proof}

Assume $A \subset B$.
We want to show $A \subset (A \cap B)$ and \underline{\hspace*{2cm}}.
The first fact is true since: $A \subset B \Rightarrow$
if $x \in A$ then \underline{\hspace*{2cm}} $\Rightarrow$
if $x \in A$ then $x \in A \textrm{ and } B$.
The second fact is true by \underline{\hspace*{2cm}}.

Conversly, assume \underline{\hspace*{2cm}}.
By the first property again, $B \supset$ \underline{\hspace*{2cm}},
so we have \underline{\hspace*{4cm}}.

\end{proof}