In the context of minimum necessary income for sponsorship, what does the term "minimum necessary income" refer to? a) The average income for urban areas of residence with 500,000 persons or more. b) The before-tax annual income necessary to support the sponsor, their family members, the sponsored foreign national, and every other person covered by a still-effective undertaking. c) The income threshold set by the sponsor's spouse or common-law partner for co-signing an undertaking. d) The annual income required to support only the sponsored foreign national and their family members