Jquery select option value checkbox

I have run into a bit of a problem which I hope someone can help me solve.

I have a searchform with some dropdown menus (A, B and C)

When I select option value B I need a checkbox to be unchecked

I have tried every solution I have found online, but I can't get it to work - I have tried to make a simple if statement but still no go.

Hope someone here can point me in the right direction.