first find if the 2 nodes are present in the tree or not
(Figure why to check this on your own)
Now,
if root is equal to B or C
ancestor found
as other value(B or C) found will be ALWAYS below this
else
do what we do in most recursion q
Lans=LCA(left)
Rans=LCA(right)
these tells us weather we have found the B or C in the left and the right subtree
if both B and C found,root is the anscestor
else NULL(think why)