Data set C has 18 integers: 4 are in the interval $[50,60)$, 6 in $[60,70)$, and 8 in $[70,80)$.

Data set D has the same shape but with intervals shifted 10 units lower: 4 in $[40,50)$, 6 in $[50,60)$, and 8 in $[60,70)$.

What is the smallest possible difference between the mean of C and the mean of D?