A 9-year-old male patient presents for an annual well check. Past medical history is unremarkable. Development has been normal. Height is 52.5 inches (133.35 cm; 50th percentile), and weight is 70 pounds (31.75 kg; 75th percentile). His BMI is 17.9. Which category does his BMI fall under?
A. Overweight
B. Obese
C. Underweight
D. Normal weight