The potential energy of a body of mass $m$ is given by $U = ax + by$,where $x$ and $y$ are the position coordinates of the particle. The acceleration of the particle is:

• A
  $\frac{(a^2 + b^2)^{1/2}}{m}$
• B
  $\frac{a^2 + b^2}{m}$
• C
  $\frac{(a + b)^{1/2}}{m}$
• D
  $\frac{a + b}{m}$