The square of Thiele modulus, $M_{T}$, is given by $M_{T}^2 = \dfrac{L^2k}{D_{eff}}$, where $L$ is the characteristic length of the catalyst pellet, $k$ is the rate constant of a first order reaction, and $D_{eff}$ is the effective diffusivity of the species in the pores. $M_{T}^2$ is a measure of

1. $\dfrac{\text{time scale of pore diffusion}}{\text{time scale of reaction}} \\$
2. $\dfrac{\text{rate of pore diffusion}}{\text{rate of reaction}} \\$
3. $\dfrac{\text{time scale of reaction}}{\text{time scale of pore diffusion}} \\$
4. $\dfrac{\text{rate of reaction}}{\text{rate of external mass transfer}}$