Extract data between two xml tag.

Xml File:

<instance ab=1 >
<a1>a1</a1>
    <a2>a2</a2>
</instance>
<instance ab=2 >
<b1>a1</b1>
    <b2>a2</b2>
</instance>
<instance ab=3 >
<c1>a1</c1>
    <c2>a2</c2>
</instance>

Looking for output by using sed or awk like following.

<a1>a1</a1><a2>a1</a2>
<b1>a1</b1><b2>a2</b2>
<c1>a1</c1><c2>a2</c2>

It's not working with following sed command.

sed -n '/<Sample/,/<\/Sample/p' ab