a$)$ Why can we not prove undecidability of $E{Q}_{CFG}$ by reduction from $E_{CFG}?$

b$)$ What is the mapping reduction that shows $AL{L}_{CFG}{?}_{m}E{Q}_{CFG}?$

c$)$ Is the $A_{TM}AL{L}_{CFG}$ reduction that a grammar that generates all strings other than accepting computation histories of $M$ on $w$ a mapping reduction $A_{TM}{?}_{m}AL{L}_{CFG}$ or $\frac{?}{\text{b}\text{a}\text{r}}(A_{TM}){?}_{m}AL{L}_{CFG}?$