A spherical ball of mass $$20$$ kg is stationary at the top of a hill of height $$100$$ m. It rolls down a smooth surface to the ground, then climbs up another hill of height $$30$$ m and finally rolls down to a horizontal base at a height of $$20$$ m above the ground. The velocity attained by the ball is

A

$$40$$ m/s

B

$$20$$ m/s

C

$$10$$ m/s

D

$$10\sqrt{30}$$ m/s