The 'or' implies that at least one is correct. As only once change has occurred we must assume that the correctness refers to the change.
OR does not imply that at least one is correct. It implies such only if the statement is true. That is, only one statement in an OR clause need to be true for the entire statement to be true (although not all statements need be true, only one of them). Thus, if the answer is "No", then all statements are false. Whereas an AND clause requires all statements to be true to be true, otherwise the entire clause is false if a single statement is false.
On the other hand I'm putting my foot down that the grammatical correctness, despite the presence of the word "sum"-- which may only be a typo, is "seven and five IS thirteen (in base 13)".
Look at these examples. Never do they ask, "What are the sum of x + y?" It's always in the format, "What IS the sum of...". Thus a little reverse engineering would enlighten our tiny brains to the truth that math uses "IS" as an equality and not "ARE".