What of the following led to a stalemate in the early years of WWI? A)the use of poison gas

B)fighter planes

C)trench warfare

D)U-boat attacks

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