I think it's letter A.litter left on the field after a ball game
<span>Ethology is a branch of zoology concerned with the study of animal behavior. Ethologists take a comparative approach, studying behaviors ranging from kinship, cooperation, and parental investment, to conflict, sexual selection, and aggression across a variety of species.</span>
They need energy to produce oxygen
The correct answer is -
E. double coincidence of wants
