  A dust scattering model was recently proposed to explain the shallow X-ray
decay (plateau) observed prevalently in Gamma-Ray Burst (GRB) early afterglows.
In this model the plateau is the scattered prompt X-ray emission by the dust
located close (about 10 to a few hundred pc) to the GRB site. In this paper we
carefully investigate the model and find that the scattered emission undergoes
strong spectral softening with time, due to the model's essential ingredient
that harder X-ray photons have smaller scattering angle thus arrive earlier,
while softer photons suffer larger angle scattering and arrive later. The model
predicts a significant change, i.e., $\Delta \b \sim 2 - 3$, in the X-ray
spectral index from the beginning of the plateau toward the end of the plateau,
while the observed data shows close to zero softening during the plateau and
the plateau-to-normal transition phase. The scattering model predicts a big
difference between the harder X-ray light curve and the softer X-ray light
curve, i.e., the plateau in harder X-rays ends much earlier than in softer
X-rays. This feature is not seen in the data. The large scattering optical
depths of the dust required by the model imply strong extinction in optical,
$A_V \gtrsim $ 10, which contradicts current findings of $A_V= 0.1 - 0.7$ from
optical and X-ray afterglow observations. We conclude that the dust scattering
model can not explain the X-ray plateaus.
