题目内容：
Passage FourUnless you have visited the southern United States, you probably have never heard of kudzu． Kudzu, as any southern farmer will sadly tell you, is a super-powered weed． It is a strong climbing vine． Once it gets started, kudzu is almost impossible to stop． It climbs to the tops of the tallest trees． It can cover large buildings． Whole barns and farm houses have been known to disappear from view． It has even been said to engulf small, slow-moving children, but that is probably an exaggeration． Still, wherever it grows, its thick, twisting vines are hard to remove．
Kudzu was once thought to be a helpful plant． Originally found in Asia, it was brought to America to help fight erosion． It was planted where its tough roots, which grow up to five feet long, could help hold back the soil． But the plant soon spread to places where it wasn't wanted． Farmers now have to fight to keep it from eating up all the nutrients in the soil and killing other plants． It has become a sign of unemployment in the South; where there is no one to work the fields, kudzu soon takes over．
The northern United States faces no threat from kudzu． Harsh winters kill off its vines． It loves the warmth of the South． But the South surely doesn't love it． If someone could invent some use for kudzu, and take it off southern farmers' lands, their fortune would be assured． This passage is mostly concerned with_ A． kudzu
B． farming
C． the South
D． soil erosion
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