单项选择题
A．measures must be taken B．people shouldn't draw benefit from trees C．government must realize the serious results D．unless trees never be cut down
A．where there are no trees, the soil becomes hard and poor B．where there are many trees, there are fewer floods C．where there are no trees, the land might become desert slowly D．floods will make the land become desert
A．to tell people that trees are very useful to man B．to warn people not to cut down trees any more C．to warn that man mustn't destroy forests any more D．to explain how trees help to prevent drought and floods