Describe a situation or financial goal in which you would choose that option over the other:

a$.$ Choose a five$-$year CD paying $4\mathrm{.}00\%$ instead of an $18-$month $CD$ paying $4\mathrm{.}50\%.$

b$.$ Choose an $18-$month CD paying $4\mathrm{.}50\%$ instead of a five$-$year CD paying $4\mathrm{.}00\%.$