Answer: Choice C. -10 + (-1)
Reason
Let's replace -11 in the original expression with -10+(-1).
-11 + 10
-10+(-1) +10
(-10+10) + (-1)
0 + (-1)
-1
The -10 and +10 pair up to cancel out. They are opposites of each other. The -10 can be thought of as going down 10 floors on a building. Then +10 is going up 10 floors. When combined, the two cancel out and you end up back on the same floor you started from.
This ultimately means -11 + 10 = -1
If you are at basement level 11, and go up 10 floors, then you'll arrive at basement level 1. Check out the diagram below to see the vertical number line diagram.