An airplane requires a longer ground roll to lift-off on hot summer days because

Q. An airplane requires a longer ground roll to lift-off on hot summer days because

(A) the thrust is directly proportional to free-stream density.

(B) the thrust is directly proportional to weight of the aircraft.

(C) the lift-off distance is directly proportional to free-stream density.

 (D) the runway friction is high on hot summer days.

Ans: the thrust is directly proportional to free-stream density.