How many four-digit positive integers a b c d, with a \neq 0, have the property that the three two-digit integers a b<b c<c d form an increasing arithmetic sequence? One such number is 4692 , where a=4, b=6, c=9, and d=2.

**Answer Choices**

A. 9

B. 15

C. 16

D. 17

E. 20