Catalytic decomposition of phosphine on hot tungsten at high pressure is ______ reaction.

A reaction  ${\mathrm{A}}_{\mathrm{2}}+{\mathrm{B}}_{\mathrm{2}}\to 2\mathrm{AB}$ occurs by the following mechanism
${\mathrm{A}}_{\mathrm{2}}\mathrm{ }\to \mathrm{A}+\mathrm{A}......(\mathrm{slow})$
$\mathrm{A}+{\mathrm{B}}_{\mathrm{2}}\to \mathrm{AB}+\mathrm{B}.....(\mathrm{fast})$
$\mathrm{A}+\mathrm{B} \to \mathrm{AB} .... (\mathrm{fast})$

Its order would be _______.