你好：为你提供精确解答
填入【is singing】
这句话是how nice the song sounds!let's see (that )who is sing.
这里是that宾语从句,做see的宾语.
切不可和see sb doing sth 混淆.
谢谢,不懂可追问how nice the song sounds!let's see (that )who[ is sing.]is singing两个答案咋不一样呢？呵呵，那个是笔误。明显是系动词+动词进行时态。sing的进行时是singing.is singing正在唱歌。 往往想尽快给提问者答案。打字不可避免差错。谢谢，不懂可追问