一、单项选择（共20小题，每小题1分，满分20分）
从A、B、C、D四个选项中，选出可以填入空白处的最佳选项。
1. I realized that was ____ airplane, not ____ UFO.
A. a; a B. a; an C. an; a D. an; an
2. ----____ do you learn to play the piano so well? ----By practicing every day.
A. How B. What C. When D. How often
3. The train arrived ______ Nanjing ________ 6:00 ________ the morning of October 9th.
A. at, at, in B. in, at, in C. in, at, on D. at, at, on
4. __________ good news they heard!
A. What a B. What C. How a D. How
