For the given digital circuit, $\mathrm{A}=\mathrm{B}=1$. Assume that $\mathrm{AND}, \mathrm{OR}$, and $\text{NOT}$ gates have propagation delays of $10 \mathrm{~ns}, 10 \mathrm{~ns}$, and $5 \mathrm{~ns}$ respectively. All lines have zero propagation delay. Given that $\mathrm{C}=1$ when the circuit is turned on, the frequency of steady-state oscillation of the output $\mathrm{Y}$ is_____________.
- $20~ \mathrm{MHz}$
- $15 ~\mathrm{MHz}$
- $40~ \mathrm{MHz}$
- $50 ~\mathrm{MHz}$