David and Kathy like to take their young son, Chaz, to Moe's Southwestern Grill for lunch after church every Sunday. While they like to eat at Moe's at any time, Sunday is particularly good because it is "kids eat free" day at the restaurant. Until Moe's began this attractive offer, Sundays were very slow. Now it is one of the busiest days of the week for the restaurant. This strategy was a way to contend with the service characteristic of what?

a. Variability.
b. intangibility.
c. Inseparability.
d. perishability.