Think of an equation:
200 - 91 = 109
You can do anything to this equation, as long as you do the same thing to both sides. So, if you add 91 to both sides, you will get:
200 = 200
Since this holds true, Susan's strategy to add her answer to 91 makes sense.
Answer:
Once upon a time, in a far away swamp, there lived an ogre named Shrek (Mike Myers) whose precious solitude is suddenly shattered by an invasion of annoying fairy tale characters. They were all banished from their kingdom by the evil Lord Farquaad (John Lithgow). Determined to save their home -- not to mention his -- Shrek cuts a deal with Farquaad and sets out to rescue Princess Fiona (Cameron Diaz) to be Farquaad's bride. Rescuing the Princess may be small compared to her deep, dark secret.
Explanation:
I think the answer is I
Since its unusual to replace (optional) with the other sentence according to reality.