If that seems surprising, it is only because of the myth that the transition to adulthood was more seamless and smoother in the past. (Para. 4)
A、If that seems surprising, it is only because people believe the story which says in the past the transition to adulthood was easier than it is now.
B、If that seems surprising, it is only because people wrongly believe that in the past the transition to adulthood was easier than it is now.