A、When the airplane slows down very quickly. B、When the airplane is making a sharp turn. C、When the pilots have a heart attack.

(5)While flying high above the Earth’s surface, jet fighter pilots may suffer loss of eyesight for brief periods. Some pilots may even lose consciousness—not being able to recognize what is happening around them. These experiences, commonly called "blackouts," can lead to tragic results.
   Researchers say two different situations can lead to a pilot’s loss of consciousness. One is "G-LOC" or G-force induced loss of consciousness, and the other one is hypoxia, which is lack of oxygen to the brain. (6) "G-LOC" happens when the airplane is making a sharp turn or speeding up very quickly. During these periods, centrifugal force reduces the normal flow of blood to the pilot’s brain. At high speed, loss of control of the aircraft has proved deadly for many pilots. (7)Hypoxia, or lack of oxygen, can affect anyone at extreme heights. That is why passenger planes are equipped with oxygen masks. Passengers are always told before flying to put on the masks in case of a sudden loss of air pressure.
5. What is the news report mainly about?
6. When does "G-LOC" usually happen?
7. Why are passenger planes equipped with oxygen masks?