clocks c and d are both good timekeepers in the sense that each is consistent in its daily drift  relative to wwf time  ; thus, c and d are easily made perfect with simple and predictable corrections. the correction for clock c is less than the correction for clock d, so we judge clock c to be the best and clock d to be the next best. the correction that must be applied to clock a is in the range from 15 s to 17 s. for clock b it is the range from 5 s to + 10 s, for clock e it is in the range from 70 s to 2 s. after c and d, a has, 