In the 1400s, European exploration was possible because of the previous technological innovations of all of the following civilizations
except
1.mongol
2.song
3.Islam
4.tang

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