18. The total weight of a box, 20 plates and 30 cups is $4.8 \mathrm{~kg}$. The total weight of the box, 40 plates and 50 cups is $8.4 \mathrm{~kg}$. What is the total weight of the box, 10 plates and 20 cups?
A $3 \mathrm{~kg}$
B $3.2 \mathrm{~kg}$
C $3.6 \mathrm{~kg}$
D $4 \mathrm{~kg}$
E $4.2 \mathrm{~kg}$