现在进行时构成 — 英语词汇题

根据句意，写出所给动词的-ing形式补全句子，注意拼写变化。1. Sam is ______ (write) a letter to his friend now. 2. Look! The birds are ______ (fly) in the sky. 3. My mother is ______ (run) on the playground. 4. They are ______ (swim) in the river.