根据句意和首字母或汉语提示，将单词的正确形式写在相应的横线上。
1. You must stop s____________, nicotine (尼古丁) is bad for your health.
2. The girl s____________ a lot of time collecting stamps last year.
3. We were glad to celebrate our motherland, China’s s__________ birthday on October 1st, 2009.
4. I like ____________ (土豆) a lot, and my mother usually cooks it in different ways.
5. Which are __________ (重的), elephants or sheep?