  In the Minimal Supersymmetric Standard Model (MSSM), the masses of the
charged Higgs boson ($H^\pm$) and the CP-odd scalar ($A$) are related by
$M_{H^+}^2=M_A^2+m_W^2$. Furthermore, because the coupling of $W^-$-$A$-$H^+$
is fixed by gauge interaction, the tree level production rate of $q \bar q' \to
W^{\pm \ast} \to A H^\pm$ depends only on one supersymmetry parameter --the
mass ($M_A$) of $A$. We show that to a good approximation this conclusion also
holds at the one-loop level. Consequently, this process can be used to
distinguish MSSM from its alternatives (such as a general two-Higgs-doublet
model) by verifying the above mass relation, and to test the prediction of the
MSSM on the product of the decay branching ratios of $A$ and $H^\pm$ in terms
of only one single parameter -- $M_A$.
