A historian presents a paper at a political science conference on the degree to which
scandals in the last year of a given presidential term affect approval ratings during
the lame duck period that ends an outgoing administration. On a screen at the front
of the room, the historian displays a bar graph of approval ratings from 1974 to the
present day. The bar graph is what type of source?
a) secondary
b) tertiary
c) primary
d) narrative