What do we call "a wave that can travel without a medium"?

The economy of the united states is best described as

Where do the majority of cases heard by the supreme court originate?

Quick using the information in the graph, one might speculate that a) w.w.i created increased job opportunities in the u.s. b) the election of president hoover in 1936 caused unemployment to decline. c) the return of soldiers following w.w.ii caused unemployment to increase in the u.s. d) the new deal programs of president roosevelt created employment opportunities in the u.s.