Which characteristic(s) birds maintain flight?

a. large heart
b. no bladder
c. air sacs
d. b and c
e. a, b, and c

answer; //the above statement is (false);

answer; i believe the correct answer to the above question is ( friction)

the answer is "a.) well–developed vascular system."

explanation:

well for one plants are always the producer, so a is the answer. a wolf is more of the consumer(predator) and the deer is the prey so i know those two are out of the question. also mushrooms are fungi/fungus so a is the producer.