​​​​​​In the following two-dimensional momentum equation for natural convection over a surface immersed in a quiescent fluid at temperature $T_{\infty}$ ($g$ is the gravitational acceleration, $\beta$ is the volumetric thermal expansion coefficient, $\nu$ is the kinematic viscosity, $u$ and $v$ are the velocities in $x$ and $y$ directions, respectively, and $T$ is the temperature) $$u\frac{\partial u}{\partial x} + v \frac{\partial u}{\partial y} = g \beta \left ( T - T_{\infty} \right ) + \nu \frac{\partial^2 u}{\partial y^2},$$ the term $g \beta \left ( T - T_{\infty} \right )$ represents

1. Ratio of inertial force to viscous force.
2. Ratio of buoyancy force to viscous force.
3. Viscous force per unit mass.
4. Buoyancy force per unit mass.