Dinosaurs became extinct in

$A$ : Mesozoic is the era of dinosaurs but they suddenly disappeared from the earth.
$R$ : Dinosaurs may have changed into birds or climatic changes killed them

The first mammals were like …$A$… Their fossils are small sized. Mammals were …$B$… and protected their unborn young inside the mother's body

Choose the correct option for $A$ and $B$ to complete the given $NCERT$ statement