根据句义和所给汉语提示完成句中所缺的单词.

1. There are four __________ (字典) on the table.

2. February is the __________ (第二) month in a year.

3. The shoes are very nice, but I can not __________ (买得起).

4. Running star Sandra eats lots of __________ (健康的)food.

5. Math is very __________ (困难的), I can not learn it well.

简述电弧炉炼钢采用废钢预热的效果。