A hypothetical reaction, $A_2+B_2 \longrightarrow 2 A B$ follows the mechanism as given below;
$A 2 \rightleftharpoons A+A \ldots . . \text { (fast) } $
$A+B_2 \longrightarrow A B+B \ldots . . . \text { (slow) } $
$A+B \longrightarrow A B \ldots . . . \text { (fast) }$
The order of the overall reaction is
(A) 2
(B) 1
(C) $1 \frac{1}{2}$
(D) zero.