Some plants have fungi that live intertwined in their roots. The fungi provide nutrients to the plants, while the plants’ roots provide a habitat for the fungi. Which relationship is demonstrated by this example

One drawback of voluntary trade is that it a.requires two groups to use a common language b.can make one group dependent upon another for necessary goods c.doesn't occur unless there is a shortage of goods for at least one group d.forces groups to rely on themselves to produce all the goods they need