I do not understand this problem.

Throughout this course we have been told that you cannot simply reverse a S&N statement. However, this question is asking us to do just that. The growing and shrinking of the ice caps is a sufficient condition for the increase and decrease of the heavy oxygen in the oceans. The "correct" answer choice shows a reversal of this statement and says that because there was an increase for 20,000 years and a decrease for 10,000 that it must be because of the icecaps growing and shrinking. Am I missing something here? Someone please help me understand.