You ought to accept Dulles for IAD since you accept Heathrow, Luten, etc. for London. There is also an airport in Washington DC called Reagan (DCA). IAD is quite a ways outside of town in Northern Virginia. I missed Luton because of spelling. :P
I kept typing Malpensa and Fiumicino and did not get the response, but with the London airports you do need to type the actual airport names. Please make this quiz more consisted!