Bones kept in hydrochloric acid becomes soft why?

Bones are made up of Calcium Phosphate given by the chemical formula, . When bones are kept in Hydrochloric acid, given by the chemical formula, HCl, they become soft because the calcium compound present in the bones changes its form. The following double displacement reaction takes place when the bones are kept in HCl,

--->

Calcium chloride is water soluble, and gets washed away. This makes the bone thin and soft.

he forelimbs below belong to four species that descended in a direct line from the ancestral mammal shown above. of the four descendants shown, which species most likely appeared earliest in evolutionary history?

the answer would be d : p