单选题 Tears are nature"s way to make us feel comfortable. When our eyes are made uncomfortable by some small pieces of pollution, or when we are cutting onions, or when we are tired and become" red-eyes" because of working for a long time, 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 (唯一) human. We know that animals also have emotion—fear, pleasure, loneliness—but they do not shed (流) tears. From this we can conclude that tears are closely related to the emotional and biological character of human.
Biologically speaking, tears are actually drops of saline fluid, which is a little bit salty (咸的), produced by a gland (腺) in the body. Because salt is an important part, tears may actually be the strongest evidence that the human is the end product of a long evolutionary process that began in the sea.
And it is clear that, the shedding of tears has a specific biological function, besides the emotional benefits (好处). Through tears, we can eliminate from our body certain chemicals which appear because of 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.