1 and d can be said to love music, but they have no professional training, no original works or original works without connotation, and their singing skills are average, so they can't control their own voices. Running a wedding team is not very demanding on singing skills, and it belongs to a D-level singer.
2. C-level singers, creative singers without singing skills, have unstable breath from a professional point of view, and singing basically depends on the natural conditions of voice. You can't sing high notes with a red face without using singing skills.
3.b-level singer. There is really no problem in singing, no matter from intonation, timbre, treble and skill, it is very good. But there is no personal style, which leads to a single style of the work.
4.a-level singers, professionals who sing songs and other vocal works are also used to calling themselves this way. As a profession, they have a standardized name "singer".