Let's examine the space issues among different degrees.

$21.$ Think about a M$-$ary tree with $120$ elements$/$keys$.$ Assume each node is full $($If $M=4,$ each node contains $3$ elements$).$

a$)$ How many bytes of memory does it take up for this tree where $M=2,3$ and $4?$ Assume each element$/$key takes up $E$ bytes and each pointer takes up $P$ bytes.

b$)$ Among these $3$ different degrees, which one takes least memory space?