On the set $R -\{ -1\} $ a binary operation $\ast$ is defined by $a \ast b = a + b + ab$ for all $a,b \in R - \{-1\}$. Prove that $\ast$ is commutative as well as associative on $R -\{-1\}$. Find the identity element on $\ast$ if any.

Question

$(A)\; 0$
$(B)\; 1$
$(C)\; 2$
$(D)\; 3 $