答案是:playing
原因如下：see,hear,watch,notice等这类的感官动词后面可以+do/doing两种用法.+do表示看见或听见做某事的全过程或经常看见或听见某人做某事,例如：I can always see some old people dance in the park.；而+doing则表示看见或听见的同时某人正在做某事,例如：I saw a little dog lying under the tree this morning.
答案是:playing
原因如下：see,hear,watch,notice等这类的感官动词后面可以+do/doing两种用法.+do表示看见或听见做某事的全过程或经常看见或听见某人做某事,例如：I can always see some old people dance in the park.；而+doing则表示看见或听见的同时某人正在做某事,例如：I saw a little dog lying under the tree this morning.