In fact, it is to specify a constant as a long type by adding an uppercase or lowercase L after the numerical value, which is the so-called long integer constant.

Correct. B is an ordinary decimal integer constant, C is an octal integer constant (starting from 0), and D is a hexadecimal integer constant (starting from 0X or 0x). These are all rules, just know them, and there is no reason to convince anyone.