1 . These books are often _______ (可获得的) at a discount. (根据汉语提示单词拼写)

2 . The threat of _______ (全球) warming will eventually force many countries to slow down its energy consumption. (根据汉语提示单词拼写)

3 . Swimming in the river is too ________ (危险的). (根据汉语提示单词拼写)

4 . I am a s ________ high school student. (根据首字母单词拼写)