In a two-dimensional velocity field with velocities u and v along x and y directions respectively, the convective acceleration along the x-direction is given by
(a) $u\frac{\partial u }{\partial x }+v\frac{\partial u}{\partial y}$ 
(b) $u\frac{\partial u }{\partial x }+v\frac{\partial v}{\partial y}$ 
(c) $u\frac{\partial v }{\partial x }+v\frac{\partial u}{\partial y}$ 
(d) $u\frac{\partial u }{\partial x }+v\frac{\partial u}{\partial y}$