Which researcher(s) argue(s) that crystallized intelligence increases in middle age, while fluid intelligence decreases?

according to classical republican political theory, which democratic republics would function best?

a.   small republics, with people who had similar interests

b.   large republics, with people who had competing interests

c.   republics made up entirely of landowners

d.   republics made up entirely of servants

i think it's b or c hope this