A, B, C, and D are the first four letters of the English alphabet. Each letter has its own unique shape and sound, which are essential for forming words. The alphabet is used in various languages and serves as a foundation for reading and writing. In addition to their phonetic roles, A, B, C, and D are often used in grading systems, where they represent different levels of performance. For example, A typically signifies excellent work, while D indicates a passing but unsatisfactory level. These letters are also used in various coding systems and classifications.