Answer: Differential Selection
Explanation: Differential Selection this is a selection threat with greatest concern when subjects cannot be randomly assigned to a treatment group, particularly if the groups had being unequal when the relevant variables were being considered before treatment intervention. By giving each participant pencils and stickers she is decreasing the threat of differential selection.
The answer is trade. So it is important for other countries to exchange currencies so that goods from other nations can be traded.
The evidence indicates that Don is exhibiting symptoms of pica.
