单项选择题
A．What chemicals tears are composed of. B．Whether crying really helps us feel better. C．Why some people tend to cry more often than others. D．How tears help people cope with emotional problems.
A．Few of them can live long. B．Few of them get along well with each other. C．Most of them live a normal life. D．Most of them differ in their likes and dislikes.
A．There was no garbage left to clean up. B．There was more garbage than before and they had to work harder. C．The fiver had become so clean that a lot of water-birds came back. D．The river was much cleaner and they had to search for garbage.