Find the Values of a and B for Which the Sequence

Multiple Choice

Find the values of a and b for which the sequence $2 , a , b , 17 , \ldots$ is arithmetic

A) $a = 7 , b = 12$
B) $a = 6 , b = 12$
C) $a = 8 , b = 12$
D) $a = 10 , b = 12$
E) $a = 8 , b = 15$