On the trip home from the meeting where this AMC10 was constructed, the Contest Chair noted that his airport parking receipt had digits of the form b b c a c, where 0 \leq a<b<c \leq 9, and b was the average of a and c. How many different five-digit numbers satisfy all these properties?

**Answer Choices**

A. 12

B. 16

C. 18

D. 20

E. 24