Do you know why all had drive partitions starts with "C"?

Do you know why all had drive partitions starts with "C"?

First-generation computers did not have hard drives to run operating systems. Therefore, the operating system was ran by floppy disks, and one or two floppy disks were used for this process. The letters "A" and "B" are allocated to those disks. It has not changed even after the advent of hard drives in computers. So all hard disk partitions still start with the letter "C".