Effects are a major problem found in cross-sectional research.

Lack of reproducibility and accuracy effects are a major problem found in cross sectional research.

Explanation:

Cross sectional research is a kind of research where the characteristics of a particular group of population at a particular time has been studied on some situation.

This research is based on the situation in which the population is placed and also on perspective of each item in the population. There is biasing problem in analyzing and concluding the results obtained from cross sectional research.

Also it is not sure that different group will show same characteristics in same situation or condition , thus accuracy in analyses and reproducibility of analysis are the major effects which causes problem in cross sectional research.

the historians cited in the article all agree with one another about the cause of the civil war. despite coming from different backgrounds and cultures, the historians agree that slavery was the root cause of the civil war. the fact that all these historiansgenerally agree with one another makes their shared viewpoints more reliable and valid, especially because the historians have different backgrounds and all work for the well-respected institutions of howard university, clemson university, and the university of colorado.

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