a. A topical drug delivery system is used for internal application, while a transdermal drug delivery system is used for external application
b. A topical drug delivery system is a type of patch, while a transdermal drug delivery system is a type of ointment
c. A topical drug delivery system delivers the drug to the skin surface, while a transdermal drug delivery system delivers the drug through the skin
d. A topical drug delivery system is a solid dosage form, while a transdermal drug delivery system is a liquid dosage form