In humans,out of $X$ pairs of ribs,only $Y$ pairs are true ribs. Select the option that correctly provides the values of $X$ and $Y$ and their explanation.

• A
  $X = 12, Y = 7$; true ribs are attached dorsally to the vertebral column and ventrally to the sternum.
• B
  $X = 12, Y = 5$; true ribs are attached to the vertebral column dorsally and to the sternum at both ends.
• C
  $X = 24, Y = 7$; true ribs are attached to the vertebral column dorsally but are free ventrally.
• D
  $X = 24, Y = 12$; true ribs are attached to the vertebral column dorsally but are free ventrally.