One of the main sources of revenue for many states is
a. a poll tax placed on voter registrations.
b. a sales tax placed on goods and services. c.a duty on products exported from the state. d.a tariff on products imported from other states.

answer; if congress didn't act on certain requests;

this null effect is caused by a weak manipulation and ineffective internal validity. in a scientific study, as the one dr. morimoto is trying to perfom, internal validity refers to the confidence placed in the cause-effect relationship and to how well an experiment is done and able to eliminate confounding variables.