How many leap years occur from 1999 through 2079, including both endpoint years?

Count multiples of 4, subtract century years not divisible by 400, and retain centuries divisible by 400. The verified count in [1999, 2079] is 20. Option A (21) is rejected because it does not equal the computed result; it represents a counting, direction, leap-year, interval, or hand-motion error. Option C (19) is rejected because it does not equal the computed result; it represents a counting, direction, leap-year, interval, or hand-motion error. Option D (22) is rejected because it does not equal the computed result; it represents a counting, direction, leap-year, interval, or hand-motion error.