Diploic bone is

Which vertebrae fuse to form the coccyx?

The number of floating ribs in the human body is (in $pairs$)

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.

Analogy type questions:
$(1)$ The first vertebra is called the $Atlas$ vertebra.
$(2)$ Bones contain $Calcium$ salts.

In which bone are the chondrocytes replaced by osteocytes during development?