A、Grassland for animals. B、A land covered with trees. C、A steep slope of a hill. D、A land with fertile soil. C短文开头提到边缘性土地土质差，一般位

(30) "Marginal" lands might have low quality soil. Or they might be on a hillside that rises sharply. Or they might be in an area that gets limited rainfall. In any case, they are the last to be planted under good conditions and the first to be abandoned under bad conditions.:
    But there are uses for marginal lands. Most often they are used as grasslands for cattle, sheep or goats. A farmer might use native grasses or non native seeds. Either way,  it is important to establish good ground cover to avoid the loss of soil through erosion. But using marginal land for grazing is not as simple as it might sound. Cattle can damage forage(草料) crops by eating down to the roots. Their weight also crushes the soil. That can make the ground too hard for growing.  (31) A way to reduce the damage from overgrazing is to move animals from one field to another. Experts say rotational grazing like this is extremely important for marginal land.
    Another use for marginal land is for tree crops. Trees help support the soil. They reduce the damaging effects of wind and rain. And they can provide grazing animals with shade from the sun. Studies have shown that white pines grow well on marginal land. They grow fast and provide good quality wood.  There are slower-growing trees like the black walnut that provide a nut crop as well as wood. Marginal lands need care. Failing to take care of them might only make a bad situation worse. But good planning can turn a marginal resource into a highly productive one.
Questions:30. Which of the following is a marginal land?
31.According to the speaker, how to reduce the damage on marginal lands from overgrazing?
32.Which of the following is NOT the advantage of planting trees on marginal land?