The main organs of the excretory system are the
large intestine and rectum
skin and kidneys
urinary bladder and skin
heart and kidneys

As weird as it may seem, its actually the skin and kidneys! the skin plays an important role with removing toxins from the body through sweat ect. and the kidneys work to neutralise toxins and break down liquids in the body, so that they can then be removed from the system and released through the body in the excretory system. 

Ithink option a is correct because you cannot donate a different blood group than yours because it won't match.option a says betsy can donate to mark. they have the same blood group so donation can happen.