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