In the numbering system described, which teeth correspond to numbers 20 and 21?

• A. Upper left central and upper left canine
• B. Lower left canine and lower left lateral
• C. Lower left second premolar and lower left first premolar
• D. Lower left first premolar and lower left second premolar

Answer: (C)

In the universal numbering system, teeth are numbered 1 through 32. The upper teeth are 1–16, and the lower teeth are 17–32. Within the lower left quadrant (numbers 17–24), the order from the back of the mouth to the front is: 17 is the lower left third molar, 18 is the second molar, 19 is the first molar, 20 is the second premolar, 21 is the first premolar, 22 is the canine, 23 is the lateral incisor, and 24 is the central incisor. So numbers 20 and 21 correspond to the lower left second premolar and the lower left first premolar, respectively. This aligns with the correct option because it matches the exact teeth identified by those numbers in this numbering system. The other options describe teeth in different quadrants or different tooth types, which do not fit these numbers.