Name the compounds whose line formulae are given below.

(A) For compound $(a)$: The longest carbon chain containing the ketone functional group $(>C=O)$ is numbered such that the ketone group gets the lowest possible locant. The chain has $7$ carbons,a ketone at position $2$,an ethyl group at position $3$,a methyl group at position $4$,and a double bond at position $5$. The $IUPAC$ name is $3$-ethyl-$4$-methylhept-$5$-en-$2$-one.
For compound $(b)$: The ring is numbered starting from the double bond such that the double bond carbons get positions $1$ and $2$,and the substituent $-NO_2$ gets the lowest possible locant. The $-NO_2$ group is at position $3$. The $IUPAC$ name is $3$-nitrocyclohex-$1$-ene.