You are answering a USMLE-style medical question. Question: After a T cell progenitor leaves the bone marrow, most of its development takes place in the thymus. It passes through several stages during its maturation. Positive selection is an important process during maturation of T lymphocytes, and it ensures the preservation of self-major histocompatibility complex (MHC)-restricted T cells. During which of the following developmental stages does a T cell undergo this process? Options: A) Pre-T cell B) Double-positive T cell C) Single-positive CD4+ T cell D) Single-positive CD8+ T cell Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.