When polar ice caps grow (during ice ages, for example), lighter forms of oxygen from water vapor and seawater accumu...

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.