The owner of a bike shop sells tricycles (3 wheels) and bicycles (2 wheels), keeping inventory by counting seats and wheels. One day she counts 13 seats and 31 wheels. How many of each type of cycle are there?

A. t + b = 13
3t + 2b = 31
B. t + b = 5
3t + 2b = 44
C. t • b = 44
3t + 2b = 13
D. t + b = 31
3t + 2b = 13