down box to display a list of the items based on what was selected in another drop down box

I need help with making this work.  I have a form with two drop down boxes: Model and Options.  I need to display only those options that are available to the model's in the downdown list.  For example, Model  #123 has 3 options available of 1 Year, 2 Year, and 3 Year

but Model #456 has 2 options available of 3 Year and 4 Year.

How do I get the Options drop down box to display the correct avaliable options based on what was selected in the Model dropdown? 

I sure could use the help!  Thanks.