核心问题
Farmland is being lost for several reasons. First, it is being built on.
句中的介词 on 的用法和意义我感到很陌生,不知道它是怎么冒出来的,而且 on 之后又没有它的宾语。
词典解答
当介词宾语变成被动句的主语时,这个介词必须保留下来(参见 agree on, look at , subject of passive sentences):
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They are building (houses) on farmland. → Farmland is being built on. (耕地上在搞建筑。)
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The nurses are taking good care of the children. The children are being taken good care of. (孩子们被很好地照料着。)