the [ ]children are singing english songs.the children are s

the [ happy]children are singing english songs.
the children are singing english songs[happily ].
the children are singing english songs because of their[ happiness].
再问： 最后一个我不明白
再答： 孩子在英文歌因为他们很幸福。 happiness做名词讲，表示“幸福”的意思.
再答： 1.development 名词形式 2.developed 发达国家 3.develops 第三人称单数 4.developing 发展中国家