An AVL tree is a Binary Search Tree that has the following additional property:

a$)$ for every node in the tree, the height of the left and right subtrees can differ

by at most $1.$

b$)$ for every node in the tree, the height of the left and right subtrees is the same.

c$)$ for every node in the tree, the height of the left and right subtrees can differ by

at most $2.$

d$)$ for every node in the tree, the height of the left subtree is twice the height of

the right subtrees.

e$)$ none of the above.