In order to set a string to reference nothing the programmer must use:

If another user has a ________ workbook open and you try to open it, the File in Use dialog box will display

Fill in the blank(s) with correct word

In ______, the left and right subtrees of any node have heights that differ by at most 1.

a) all trees b) all binary tress c) n-ary trees d) balanced binary trees