<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A young researcher is studying the structure of class I and class II major histocompatibility complex MHC molecules. He understands that these molecules are proteins, but the structures of class I MHC molecules are different from those of class II. Although all these molecules consist of α and β chains, some of their domains are polymorphic, meaning they are different in different individuals. He calls them P domains. The other domains are nonpolymorphic, which remain invariant in all individuals. He calls these domains N domains. Which of the following are examples of N domains?? A: β2-microglobulin in class I molecules and β1 domain in class II molecules, B: α1 domain in class I molecules and α1 domain in class II molecules, C: α2 domain in class I molecules and β2 domain in class II molecules, D: α1-α2 domains in class I molecules and α1-β1 domains in class II molecules, E: α3 domain in class I molecules and β2 domain in class II molecules,  <response>: This is a question about the structure of MHC class I and class II molecules. It provides background that these molecules consist of alpha and beta chains, some domains are polymorphic P domains while others are nonpolymorphic N domains. We need to identify examples of the invariant N domains in these molecules.To answer this, we need to know the basic structure of MHC class I and class II molecules. Class I consists of a polymorphic alpha chain alpha 1 and alpha 2 domains along with beta-2 microglobulin. Class II consists of a polymorphic alpha chain alpha 1 domain and a polymorphic beta chain beta 1 domain. The nonpolymorphic domains are alpha 3 in class I and beta 2 in class II.So we need to look for answer choices that mention these invariant alpha 3 and beta 2 domains.E: α3 domain in class I molecules and β2 domain in class II molecules 