Let $\Sigma=\{a, b, c, d\}$ and let $L=\left\{a^i b^j c^k d^l \mid i, j, k, l \geq 0\right\}$.

Which of the following constraints ensure(s) that the language $L$ is context-free?

Consider the following context-free grammar $G$ :

$$ \begin{aligned} & S \rightarrow a b a A B A b b a \\ & A \rightarrow a a B B A b \mid b B a b a a \\ & B \rightarrow a B b \mid a b \end{aligned} $$

In the above grammar, $S$ is the start symbol, $a$ and $b$ are terminal symbols, and $A$ and $B$ are non-terminal symbols.

Let $L(G)$ be the language generated by the grammar $G$. For a string $s \in L(G)$, let $n_1(s)$ be the number of a's in $s$ and $n_2(s)$ be the number of b's in $s$.

Which of the following statements is/are true?

Consider the following two languages over the alphabet $\{a, b, c\}$, where $m$ and $n$ are natural numbers.

$$\begin{aligned} & L_1=\left\{a^m b^m c^{m+n} \mid m, n \geq 1\right\} \\ & L_2=\left\{a^m b^n c^{m+n} \mid m, n \geq 1\right\} \end{aligned}$$

Which ONE of the following statements is CORRECT?

Consider a context-free grammar $G$ with the following 3 rules.

$S \rightarrow aS, \ S \rightarrow aSbS, S \rightarrow c$

Let $w \in L(G)$.

Let $n_a(w)$, $n_b(w)$, $n_c(w)$ denote the number of times $a$, $b$, $c$ occur in $w$, respectively. Which of the following statements is/are TRUE?