You are answering a USMLE-style medical question. Question: A researcher is testing an unknown compound, X, and seeing the effect it has on renal function. When she applies compound X to the cells, she finds that the urinary concentration of sodium and the pH decreases while the urinary potassium increases. Which of the following correctly states: 1) the cells of the kidney this compound acts on and 2) the identity of compound X? Options: A) Distal convoluted tubule; atrial natriuretic peptide B) Distal convoluted tubule; hydrochlorothiazide C) Principal cells; aldosterone D) Principal cells; spironolactone Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.