B$-$tree of order $n$ is a n$-$wyy search tree in which each non$-$root node contains

A$.$ Exactly $\frac{n+1}{2}$ krys

B$.$ exactly $\frac{n-1}{2}kgys$

C$.$ at least $2n$ keys

D$,$ at least $[\frac{n}{2}]-1$ keys