Identify the Equilibrium Expression for the Following Reaction, Assuming Homogeneity $\leftrightarrows$

Identify the equilibrium expression for the following reaction, assuming homogeneity:
Boy + girl $\leftrightarrows$ Couple

A) $K = \frac { [ \text { boy } ] [ \text { girl } ] } { [ \text { couple } ] }$
B) $K = \frac { [ \text { boy } ] + [ \text { girl } ] } { [ \text { couple } ] }$
C) $K = [ \text { boy } ] [ \text { girl } ] [ \text { couple } ]$
D) $K = \frac { [ \text { couple } ] } { [ \text { boy } ] [ \text { girl } ] }$
E) $K = [ \text { couple } ] - [ \text { boy } ] - [ \text { girl } ]$