One reason for using random assignment is: a) to make sure that participants follow the rules of the study b) to ensure that participant characteristics don't become confounding variables c) to give the experimenter control so that he/she can find significant results d) it is a required component of all psychology studies.

b) to help ensure that participant characteristics don't become confounding variables

Explanation:

In psychology, the term Random assignment refers to the use of procedures involving just chance when creating groups for a study so that each participant has the same opportunity to be assigned to any group. In other words, the participants get to be randomly assigned to the groups and therefore creating a more valid experiment. Since the participants are randomly assigned, this method helps ensure that the characteristics of the participants don't affect the results of the study (since they were randomly assigned)

Therefore, one reason for using random assignment is b) to help ensure that participant characteristics don't become confounding variables

what is the tax base for the sales tax?

it should be (usually ) 6%

i would say b. research typically uses a variety of sources