5.recent cross-national studies looking at academic performance have found that . a.academic performance in the 4th grade is related mainly to countries' cultural beliefs rather than to economic development b.academic performance in the 4th grade is related mainly to countries' length of school day spent on music and sports c.academic performance in the 4th grade is related mainly to countries' economic development rather than to differences in cultural beliefs d.academic performance in the 4th grade is related mainly to a child's innate ability