Consider the premise. Does the hypothesis stand independently without contradicting or following it? Premise: At 8:51, the controller noticed the transponder change from United 175 and tried to contact the aircraft. Hypothesis: The transponder code on United 175 stayed the same.
At 8:51, the controller noticed the transponder change from United 175 and tried to contact the aircraft.###The transponder code on United 175 stayed the same.