Desmond must enter a long string of numbers (1 4 9 2 1 6 2 0 1 7 7 6) into an old computer every two hours or the computer will mysteriously shut down. desmond finds that combining the numbers into three meaningful years (1492, 1620, 1776) makes it easier for him to remember the string. desmond is relying on which memory aid to help him recall the numbers?