A、The jet was intercepted by US air force. B、The plane’s transponder mistakenly transmitted code. C、The pilots repeatedly told c

　　A passenger jet headed for New York City was forced to land at Halifax, Canada this morning. (3) The plane’s transponder was broadcasting a signal indicating it had been hijacked.
　　The Virgin Atlantic flight from London’s Heathrow Airport was midway across the Atlantic when air traffic control officials in the US started receiving its signal: a signal which carried a special code indicating there was a hijacking in progress.
　　Canadian fighter jets operating under NORAD, the joint Canada-US command, were scrambled to intercept the jet.
　　According to NORAD, the pilots of the passenger jet repeatedly radioed in to tell controllers they were not being hijacked. But US officials erred on the side of caution. After the plane landed, armed police boarded it to ensure there were no hijackers. (4) It was determined that the plane’s transponder had mal-functioned, and was mistakenly transmitting code "7500" for hijacking.
　　3.  Why was the passenger jet forced to land at Halifax, Canada?
　　4.  What has really happened to the passenger jet?