\label{eq:OT1}\end{aligned}$$

From the experimental ratio $[d\Gamma   (D\rightarrow   \rho
\bar{\ell}  \nu_{\ell}  )/dq^{2}] /[d\Gamma  (D\rightarrow  K^{*}
\bar{\ell} \nu_{\ell} )/dq^{2}]_{q^{2}  \rightarrow 0}$ one needs knowledge of $|A^{D\rightarrow   \rho}_{0}(0)|/|A^{D\rightarrow
K^{*}}_{0}(0)|$ to extract the ratio $|V_{cd}/V_{cs}|$. This ratio is often taken to have the value unity by $SU(3)$–flavor symmetry.

There have been many model–dependent studies of this ratio which we show in table \[tab1\]. From table \[tab1\], we see that theoretical predictions of the ratio of form-factors fall in a range near $1$. The procedure adopted to get $A_{0}(0)$ often uses Eq. (\[def\]) with the calculated values of $A_{1}$ and $A_{2}$ rather than $A_{0}$ directly. However, this indirect way to get $A_{0}$ may have some difficulties coming from the $q^{2}$ dependences of the form factors and also from possible correlations in treating the errors. This has already been commented on in ref. [@CFS] and we will have further remarks to make when we discuss the non-massless lepton case.

We use the light-front quark model, which is suitable at the kinematic limit where $q^{2}=0$, to determine the same ratio. This model was developed [@model] a long time ago and there have been many applications [@Jaus90]-[@GuHu] where the details can be found.

In the light -front quark 