The grade of concrete M 20 means that characteristic compressive strength of 15 cm cubes after 28 days is given by

A

$$10 \frac{N}{mm^{2}}$$

B

$$15 \frac{N}{mm^{2}}$$

C

$$20 \frac{N}{mm^{2}}$$

D

$$25 \frac{N}{mm^{2}}$$