Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: Red blood cells ( Figure below ) are made in the red marrow of long bones, rib bones, the skull, and vertebrae. Each red blood cell lives for only 120 days (about four months). After this time, they are destroyed in the liver and spleen. Mature red blood cells do not have a nucleus or other organelles. Lacking these components allows the cells to have more hemoglobin and carry more oxygen. Answer: carry more oxygen
A:
Red blood cells don't have a nucleus. this allows them to do what?