现在进行时描述正在进行的动作 — 英语填空题
题目
用所给动词的现在分词形式补全句子（注意分词的构成规则）。1. Look! The boys are ___ (swim) in the river. 2. She is ___ (write) a letter to her friend now. 3. Listen! They are ___ (sing) an English song. 4. He is ___ (run) to catch the bus. 请写出每空所填单词。