Hello Lsatmax!

Hi Masada, thanks for the question! Funny enough that you should ask about A, but A is discussing a contrapositive. If there is a necessary condition established for something, then it always holds that without this necessary condition we cannot have that something. It describes the following: A -> B; B is necessary. Without B, A cannot hold NOT B -> NOT A This is not an error. In fact, contrapositives are always valid from their original conditional statement. I hope this helps. Please let me know if you have any other questions or would like me to elaborate further!

Thank you so much Ben !