The correct answer is option 4: to distinguish between two kinds of sounds arriving at the ears in opposite directions. Having two ears allows humans to perceive and differentiate between sounds coming from different directions. This is important for our survival as it helps us identify potential threats or sources of danger. For example, if a loud noise is coming from one side, we can turn towards it and assess the situation. Additionally, having two ears enables us to localize sound sources accurately, which is important for communication, hunting, and navigating in our environment. While options 1, 2, and 3 also have some truth to them, the primary function of having two ears is to differentiate between sounds arriving from different directions.