输入一段英文文本，将其中的所有复数名词转化为单数形式。
I saw a lot of trees when I went for a walk in the park. The children were playing with their toys and the dogs were barking at the squirrels.
I saw a lot of tree when I went for a walk in the park. The child was playing with their toy and the dog was barking at the squirrel.