问题描述：
The floor of the attic needs ____if we are going to make it into a bedroom.空白为什么只能...
The floor of the attic needs ____if we are going to make it into a bedroom.空白为什么只能填strengthening而不能填strengthened?
The floor of the attic needs ____if we are going to make it into a bedroom.空白为什么只能填strengthening而不能填strengthened?
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