September 11, 2001 is the day ______ we will never forget.
Some people ________cruel experiments on animals ________science.
—Did you remember to give Mary the money you owed her?—Yes. I gave it to her ________I saw her.
_____ to finish this composition in such a short time is a mystery.
Einstein’s most famous theory is ______ we call the Theory of Relativity.
Not everyone has realized what ________important part the balance of ________nature plays in their life.