—The 2010 World Expo of Shanghai is ___ first time for the world to be hold in developing countries ---Yes. All of us want to have ___ second time to hold it.
—I won't get good grades_______ I study hard.
—That's ______ everyone is trying his best to prepare for the exam.
A. until; why B. unless; when C. unless; why
—I don't know if Sam ______ tomorrow.
—Don't worry. I will tell you as soon as he ______.
A. comes; will come
B. will come; comes
C. will come; will come