The co-ordinates of a moving particle at any time $t$ are given by $x = \alpha t^3$ and $y = \beta t^3$. The speed of the particle at time $t$ is given by :

Question

( A ) $ \sqrt{\alpha^2+\beta^2}$
( B ) $3t \sqrt{\alpha^2+\beta^2}$
( C ) $3t^2 \sqrt{\alpha^2+\beta^2}$
( D ) $t^2 \sqrt{\alpha^2+\beta^2}$