Which groups of people were imprisoned in concentration camps? Choose all answers that are correct.

people with disabilities or mental illness

Communist party members

people from so-called "inferior" races

Nazi party members

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