According to developmental psychologist K. Warner Schaie, numeric ability tends to increase until the
a) mid-50s, is lower at age 60, and then stays steady.
b) mid-40s, is lower at age 60, and then stays steady.
c) mid-30s, is lower at age 60, and then stays steady.
d) mid-20s, is lower at age 60, and then stays steady.