What Kind of Cultural Study: Within-Cultural Comparison or Cross-Cultural Comparisons

What kind of cultural study: within-cultural comparison or cross-cultural comparisons makes the most sense?

A) Both can make sense, it depends on what theory you are testing.
B) Both can make sense, although one should study within-cultures most often.
C) Neither makes sense because this type of research is impossible to do.
D) Neither makes sense as both are difficult to accumulate statistical data for.