Get and then use the selected option from a select menu

Get and then use the selected option from a select menu

Hello,
This is my first post. Hopefully this will be a really easy question...

This is my HTML...

<select blah... >
<option select="selected">Choose a company</option>
<option value="comp1">Company One</option>
<option value="comp2">Company Two</option>
<option value="comp3">Company Three</option>
</select>


If a user selects 'Company One' (not submit, just the action of selecting) then a button elsewhere on the page is given the class "hide" and another select menu has the class "hide" removed.

I'm fine with adding and removing classes, I just need a hook.

Many thanks in advance...

Hope you can help.