executing threads with no synchronization
Hint: Let $C = B \pm A$ be the final instruction being executed. Clearly $A \in \{3,4\}$ and $B \in \{5,6\}$, and this shows that $C \in \{1,2,3,8,9,10\}$. For the other direction, show that by carefully scheduling both threads, you can choose which of $C = B \pm A$ is executed last, and choose the value that $A$ and $B$ get prior to that instruction being executed.