A cantilever beam 6 meter long as shown in figure is subjected to a linearly varying loading which has a maximum ordinate of 360 N/m at the fixed end on the right. The moment as a function of x is

A

$$M = -3x^3$$

B

$$M = -6x^3$$

C

$$M = -10x^3$$

D

$$M = -20x^3$$