Although Abel felt that random assignment to treatment groups would be more scientifically credible than would be self-selection of participants, he allowed for participants to choose to be part of a treatment group. This sort of concern for participants creates internal validity problems for the research, but demonstrates what sort of approach or orientation:

Quasi-experimental approach

Explanation:

The experiment presented above represents a quasi-experimental approach. This type of approach allows the experiment to be established, without the sample used presenting a random distribution, as was done in the experiment presented above. As randomness is ruled out, researchers can assign the formation of treatments and manipulate the composition of experimental groups, but to maintain the credibility of the experiment, it is necessary for the researcher to use an eligibility cut-off mark instead of random assignment.

the answer is:

c. economic output