Read the following paragraph along with the question and answer. Then, respond with whether the answer is correct.
Passage: American Airlines Flight 11: FAA Awareness. Although the Boston Center air traffic controller realized at an early stage that there was something wrong with American 11, he did not immediately interpret the plane's failure to respond as a sign that it had been hijacked. At 8:14, when the flight failed to heed his instruction to climb to 35,000 feet, the controller repeatedly tried to raise the flight. He reached out to the pilot on the emergency frequency. Though there was no response, he kept trying to contact the aircraft. At 8:21, American 11 turned off its transponder, immediately degrading the information available about the aircraft. The controller told his supervisor that he thought something was seriously wrong with the plane, although neither suspected a hijacking. The supervisor instructed the controller to follow standard procedures for handling a "no radio" aircraft. The controller checked to see if American Airlines could establish communication with American 11. He became even more concerned as its route changed, moving into another sector's airspace. Controllers immediately began to move aircraft out of its path, and asked other aircraft in the vicinity to look for American 11. At 8:24:38, the following transmission came from American 11: American 11: We have some planes. Just stay quiet, and you'll be okay. We are returning to the airport. The controller only heard something unintelligible; he did not hear the specific words "we have some planes." The next transmission came seconds later: American 11: Nobody move. Everything will be okay. If you try to make any moves, you'll endanger yourself and the airplane. 

Question: What circumstances prompted tower control to contact American Airlines Flight 11? How was the pilot contacted?

Answer: The aircraft failed to heed instructions to increase altitude, the pilot was contacted on emergency frequency

Is the answer correct? Respond with only "Yes" or "No" and no additional text.
Yes
Yes.