$(4)(15$ pts$)$ Construct the CFG that generates $|=j$ or $\{$:$j=k\}.$

$(5)(5$ pts each$)$ Show that the following grammars are ambiguous by demonstrating two different left$-$most derivations.

$($a$)SSS|a|b$

$(b)SA|B|$

$A$aAb$|ab|$

$B$abB$|C|$