What Is the Primary Challenge to Acid-Base Homeostasis

What is the primary challenge to acid-base homeostasis?

A) The primary challenge is the difficulty exhaling CO₂ causing accumulation of acid.
B) The primary challenge is that buffer systems only work within tissues not in the blood plasma.
C) The primary challenge is that blood pH must be maintained at exactly 7.0.
D) The primary challenge is that the body generates a variety of acids during normal metabolic operations causing a decrease in pH.
E) The primary challenge is that only the lungs can increase pH.