**Core Concept**
Both T cells and B cells are central components of the adaptive immune system, playing crucial roles in recognizing and eliminating pathogens. T cells and B cells undergo distinct but related developmental processes, involving the selection and maturation of lymphocytes in the thymus and bone marrow, respectively. The developmental pathways of T cells and B cells share several similarities, but also exhibit notable differences.

**Why the Correct Answer is Right**
Positive selection is a process that occurs during the development of T cells, specifically in the thymus. It involves the selection of T cells with functional T cell receptors (TCRs) that can recognize self-MHC molecules. This process ensures that mature T cells can interact with antigen-presenting cells (APCs) displaying self-MHC-peptide complexes. In contrast, B cells undergo a process called negative selection in the bone marrow, where B cells with receptors that recognize self-antigens are eliminated to prevent autoimmunity. Therefore, positive selection is not shared between T cells and B cells.

**Why Each Wrong Option is Incorrect**
**Option B:** Class I MHC Expression is shared between T cells and B cells, as both express class I MHC molecules on their surface to present peptides from proteins synthesized within the cell to cytotoxic T cells.

**Option C:** Antigen-specific receptors are indeed shared between T cells and B cells, as both T cells (TCRs) and B cells (BCRs) express receptors that recognize specific antigens.

**Clinical Pearl / High-Yield Fact**
T cells undergo positive selection in the thymus, whereas B cells undergo negative selection in the bone marrow, highlighting the distinct developmental pathways of these two lymphocyte populations.

**✓ Correct Answer: A. Positive Selection during Development**