Cardio coasting is detrimental to a fitness routine because
a. the lack of variation in activity can lead to boredom and eventually cause exercise to stop.
b. it can cause extreme weight gain.
c. it can increase the chance of injury.
d. it can increase the chance of a heart attack.

I chose a. the lack of variation in activity can lead to boredom and eventually cause exercise to stop. Your body will get use to the same routine and would no longer be pushed to another level.  You have to try new exercises that would boost your cardio without necessarily overworking yourself.