When a researcher asks the question, "Have subjects been pre-selected so as to represent the extremes on the variable of interest?", she is concerned with which one of the following potential threats to internal validity?

A. Regression effect
B. Subject selection
C. Testing effect
D. Maturation

would it be plagiarism to copy and paste off of here? ?

this is called police cynicism. i hope this .

not  

cuba  

russia  

iraq  

el salvador  

explanation:

known as the land of volcanoes, el salvador has visit seismic tremors and volcanic movement. it is the main nation in central america that does not have a coastline on the caribbean sea. about portion of all salvadorans live in the wide open. they are poor and don't have power or running water in their homes. in 1525, pedro de alvarado, a lieutenant of cortés's, vanquished el salvador. el salvador, with alternate nations of central america, pronounced its freedom from spain on sept. 15, 1821, and was a piece of a league of central american states until the point when that association broke up in 1838.

answer; archaeologists;