In working a Sudoku (a Japanese type of logic problem), solvers must determine which of the digits 1 through 9 belongs in each square in a grid compose of nine subgrids

Each row, column, and subgrid must have all of the digits 1 through 9, and digits may not be repeated in any row, column, or subgrid. In these puzzles, which of the following acts would correspond to pruning the search tree?
a) Ruling out a number in a particular square because it has previously occurred in that row
b) Making sure that the number 7 appears in every subgrid.
c) Searching for digits that could possibly go in any one of the unsolved squares.
d) Placing the last of the nine digits in one of the subgrids.

A