By applying the three production rules by replacing an occurrence of the nonterminals on the left-hand side of the production rule with those that appear on the right-hand side, the string "atattattatt" can be derived from the non-terminal S by constantly applying a series of derivations: S → ABBB → atBBB → atAtBB → atattBB → atattAtB → atattattB → atattattAt → atattattatt.