NEW YORK (AP) — A decade after 9/11, there are still more questions than answers about the health problems afflicting thousands of people exposed to dust from the World Trade Center.     Tons of gray dust fell on the city after the twin towers collapsed. In the years since, people have blamed everything from skin cancer, to vitamin deficiencies, to strange rashes on the ash and smoke at the site.     But to date, the list of illnesses that scientists have been able to link to the disaster with any confidence is relatively short.     Doctors monitoring ground zero workers say asthma and chronic sinus problems are the two most common illnesses they have found so far.     A large number of workers also say they suffer from post-traumatic stress disorder.