There are two types of hot-water heaters: tank and inline. an experiment is to be conducted to determine whether the life of the hot-water heater is the same for both types of hot-water heaters. in previous studies, it was determined that family size (small, medium, large) is associated with life expectancy of the water heater, but house size (small, medium, large) is not associated with life expectancy of the water heater. how would blocking best be used?