Selective polishing is recommended, because _ .
a. the abrasive agent used during coronal polishing actually removes a small amount of the fluoride-rich outer enamel layer
b. the abrasive agent used during coronal polishing actually removes a small amount of the fluoride-depleted outer enamel layer
c. the abrasive agent used during coronal polishing impedes uptake of fluoride by the outer layer of enamel
d. the fluoride contained in the abrasive agent is not taken up by the teeth