After scoring a touchdown in football, the scoring team is entitled to either earn 1 extra point (by kicking the ball through goalposts) or 2 extra points (by advancing the ball past the goal line). Up until the 2016-2017 football season, teams who attempted to earn 1 extra point had a success rate of 0.993; teams who attempted to earn 2 extra points had a success rate of 0.480.

a) Use the concept of expected value to explain why teams would typically go for the extra point.

b) Use the concept of expected value to explain the justification in moving the snap for an extra point back to the 15-yard line after scoring a touchdown in professional football. HINT: The success rate of kicks where the snap is at the 15-yard line is 0.959.

Please be clear and detailed in the explanation, and provide at least one source/referenece to support your responses.