Let $\sigma _{v}^{'}$ and $\sigma _{h}^{'}$ denote the effective vertical stress and effective horizontal stress, respectively. Which one of the following conditions must be satisfied for a soil element to reach the failure state under Rankine's passive earth pressure condition?

- $\sigma _{v}^{'} < \sigma _{h}^{'}$
- $\sigma _{v}^{'} > \sigma _{h}^{'}$
- $\sigma _{v}^{'} = \sigma _{h}^{'}$
- $\sigma _{v}^{'} + \sigma _{h}^{'} = 0$