Tears are nature's way of making us feel more comfortable. When our eyes is made uncomfortable by some small pieces of pollution, or when we are cutting onions, or when we are exhausted and "red-eyes" from over work and late hours, tears form in our eyes to clean and refresh them. Tears are also a sign of strong emotion . We cry when we are sad and we cry when we are happy. And tears seem to be uniquely human. We know that animals also experience emotion fear, pleasure, loneliness, but they do not shed   tears. Biologically speaking, tears are actually drops of saline fluid, which is a little bit salty, produced by a gland   in the body. Because salt is an important component, tears may actually constitute   the most conclusive evidence that the human animal is the end product of a long evolutionary process that began in the sea. And it is clear that, in addition to the emotional benefits, the shedding of tears has a specific biological function as well. Through tears, we can eliminate from our body certain chemicals which build up in response to stress and create a chemical imbalance in the body. Crying actually makes us feel better by correcting that imbalance and making us feel good again. And thus the emotional and the biological functions of tears merge   into one and make us even more "human" than we would otherwise be. According to the article, which of the following is unique to humans?
The feeling of loneliness. The state of feeling good. The ability to shed tears. The feeling of fear.