2. (10 points) Xiao Ming wrote 6 numbers on the front and back of three cards, with one number on each side, and the sum of the two numbers on each card is equal. Then he placed the cards on the table and found that the numbers on the front were $28, 40, 49$, and the numbers on the back could only be divided by 1 and themselves. Therefore, the average of the three numbers on the back is ( )
A. 11
B. 12
C. 39
D. 40
【Analysis】This problem tests the divisibility characteristics of numbers.
【Solution】Solution: Since $28, 40, 49$ have different parity, according to the condition that the sum of the two numbers on the front and back of the card is equal, the number on the back of 49 is 2, and the sum is $49+2=51$,

thus the average of the numbers on the back is $(51 \times 3-28-40-49) \div 3=12$.