单词拼写;根据下列句子及所给单词的首字母，写出各单词的完全形式(每空限填一词）。
1. You’d better be s_______, we’re not joking.
2. Tree Planting Day is on M______ 12th.
3. You shouldn’t leave a child a__________ in the house. It’s dangerous.
4. He was so sad at the bad news that he could h______ say a word.
5. The cake smells awful, you’d better t_____ it away.
6. Chinese is s______ by the largest number of people in the world.
7. The air in countryside is far f______ than that in big cities.
8. H______ of the students will take part in the activity this week .
9. The question is very simple, but n______ can answer it.
10. We will have a picnic u________ it rains tomorrow.