Out of 'X' pairs of ribs in humans only 'Y' pairs are true ribs. Select the option that correctly represents values of X and Y and provides their explanation :

A

X = 24, Y = 7 True ribs are dorsally attached to vertebral column but are free on ventral side

B

X = 24, Y = 12 True ribs are dorsally attached to vertebral column but are free on ventral side

C

X = 12, Y = 7 True ribs are attached dorsally to vertebral column and ventrally to the sternum

D

X = 12, Y = 5 True ribs are attached dorsally to vertebral column and sternum on the two ends