4. A and B are married and own an apartment together. A purchased the apartment for $1,000,000 and titled it as A and B joint tenants with rights of survivorship. A and B lived in the apartment as their principal residence from 2020 to 2023. A and B moved to Paris in 2024. A is a U.S. citizen, B is not. A and B sold the apartment in 2025 for $5,000,000. a. What income gain or loss do A and B recognize in 2025