19. Bicarbonate crosses the apical membrane of the proximal tubule by

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(A) $$\text{HCO}_3^- / \text{Cl}^-$$ countertransporters.
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(B) $$\text{Na}^+/\text{HCO}_3^-$$ cotransporters.
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(C) conversion to carbon dioxide.
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(D) $$\text{HCO}_3^-$$ active transporters.
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(E) $$\text{Na}^+/\text{HCO}_3^-$$ countertransporters.