Why is "C" the default drive on Your Computer?

Have ever thought "Why is "C" the default drive on Your Computer (Instead A or B) ?" 

Floppy disks

Hard disk

• We have been using standard hard disks since 1980
• Before that Floppy disks were used to store the data,instead hard drives.
• Floppy disks existed in 1960
• Floppy disks were available in two sizes. i.e. 5 1/2 '' and 3 1/4 '', and those were labeled with  A and B.
• Hard disc drivers augmented and then replaced the floppy disks. Hence the letter C was used.