I have a form with several drop down lists. I want to use javascript to deactivate a certain drop down list when a certain option is selected. Take a look at the example below. In a form I have the following drop down lists:

Which form of transport do you prefer?
<select name="transport">
<option value="Airplane">Airplane
<option value="Train">Train
<option value="Boat">Boat
<option value="Car">Car

How many miles do you drive per year?
<select name="miles">
<option value="15000-">Less than 15,000
<option value="15000+">More than 15,000

But the second drop down list should be deactivated when someone selects a transport other than "Car". Does any one know how to do this with javascript?