In order for the kidney tubules to continue to secrete hydrogen ions (H+) into the tubular filtrate, ...

In order for the kidney tubules to continue to secrete hydrogen ions (H+) into the tubular filtrate, mechanisms must exist that allow the secreted hydrogen ions to be taken up (buffered) by components in the tubular filtrate.
 
  One such mechanism is: A. Secretion of ammonia (NH3) by tubular epithelial cells that combines with the hydrogen ions to form ammonium ions (NH4+)
  B. Secretion of bicarbonate ions (HCO3) into the tubular filtrate by the tubular epithelial cells
  C. Secretion of phosphate ions into the tubular filtrate by the tubular epithelial cells
  D. Excretion of urea into the tubular filtrate