For a string $x=x_1 \cdots x_n \in \Sigma^*$, where $\Sigma$ is any alphabet and $x_1, \ldots, x_n \in \Sigma$, we write $x^{\uparrow m}=x^m$ (that is, the usual power of strings) and $x^{\downarrow m}=x_1^m \cdots x_n^m$.
For empty string $x=\varepsilon, \varepsilon^{\uparrow m} =\varepsilon ; \varepsilon^{\downarrow m} =\varepsilon $.
Which of the following languages cannot be described as "context-free but not regular"? (Assume $\Sigma=\{a, b, c\})$
- $\left\{x^{\downarrow 2} \mid x \in \Sigma^*\right\} $
- $\left\{(a b c)^{\uparrow n} \mid n \geq 0\right\}$
- $\left\{x^{\uparrow 2} \mid x \in \Sigma^*\right\}$
- $\left\{(a b c)^{\downarrow n} \mid n \geq 0\right\}$
