Select the correct statement$($s$)$ regarding IEEE $802\mathrm{.}15$ core and adopted protocols.

a$.$ both core and adopted protocols must be used in its entirety in an WPAN application

b$.$ adopted protocols are used to physically connect various BT devices, while core protocols operate exclusively at the upper application layers

c$.$ parts of the core specifications are optional to better enable device customization

d$.$ all statements are correct