$($a$)$ Let $A=\{1,2,3,4\},B=\{3k|kinZ-\{-1,1\}\}$ and $C=\{ninZ|n^2+n=0\}.$

Which one of the following statements is True? Explain your answer

i$.AB=\frac{O}{?}$

ii$.B-C=\frac{O}{?}$

iii. $BC=\frac{O}{?}$

iv$.AC=\frac{O}{?}$