单项选择题
A．seldom B．often C．never D．always
A．Some stamps have a flower or animal on one side. B．Along with a picture on one side, there are also some words. C．Some used stamps can be worth a lot of money. D．A stamp, when used, is worth more than a new one.
A．5 per cent of the world’s water B．15 percent of the world’s water C．20 percent of the world’s water D．50 percent of the world’s water