Identify one of the two nitrosoureas classified as alkylating agents.

Carmustine is a nitrosourea, classified as an alkylating agent, used in the treatment of various types of cancer, including brain tumors and lymphoma. Alkylating agents work by adding alkyl groups to the DNA molecule, which can result in DNA damage and ultimately lead to cell death. Specifically, nitrosoureas are a subclass of alkylating agents that have a distinctive chemical structure that enables them to cross the blood-brain barrier, making them particularly effective for tumors located in the central nervous system.

In contrast, the other options listed are not classified as alkylating agents. Fluorouracil, Gemcitabine, and Capecitabine belong to different classes of chemotherapy drugs. Fluorouracil is an antimetabolite, targeting the synthesis of DNA and RNA. Gemcitabine is also an antimetabolite and is specifically a nucleoside analog that interferes with DNA synthesis. Capecitabine, a prodrug of fluorouracil, also acts as an antimetabolite. Therefore, Carmustine stands out as the correct choice in this context due to its unique mechanism and classification among the mentioned options.