Identify the incorrect statement with regard to the Canada Pension Plan $($CPP$).$

a$.$The CPP retirement benefit is reduced by $0\mathrm{.}6\%$ for every month if taken before the age of $65.$

b$.$The contributory period starts at age $18$ and can be extended to age $70.$

c$.$A drop$-$out provision covers the months the contributor was raising children under the age of $7.$

d$.$The year$$s basic exemption $($YBE$)$ is $$3,500$ and is indexed to inflation every year.