jmc

Average the differences between each day. We get $10,-10,20,30,-20$. We find the average of this list to get $\boxed{\text{(A)}6}$. ( In case you were wondering, the way to calculate the average is $\frac{(10+(-10)+20+30+(-20))}{5}=\frac{30}{5}=6$. So the answer is indeed $\boxed{6}$)