When an Individual's Serum PH Begins to Fall Below 7

When an individual's serum pH begins to fall below 7.35, the mechanism that responds instantaneously to return the serum pH to normal level is

A) ammonia gas combining with H+.
B) excretion of acid by the lungs.
C) plasma bicarbonate buffering the acid.
D) secretion of hydrogen ions (H+) by the kidneys.