The elderly patient is diagnosed with chronic glomerulonephritis. Which lab value indicates the condition has gotten worse?

A. The BUN is 15 mg/dL.
B. The creatinine level is 1.2 mg/dL.
C. The glomerular filtration rate is 40 mL/minute.
D. The 24-hour creatinine clearance is 100 mL/minute.