What are these?They have 3 eyes,red,yellow and green.Oh,they aretraffic light.But why do they have 3 colours?Because the
问题描述：
What are these?They have 3 eyes,red,yellow and green.Oh,they aretraffic light.But why do they have 3 colours?Because there ary_______ ________.
这是完整的题目,不知道横线上填什么啊,
答
What are these?They have 3 eyes,red,yellow and green.Oh,they aretraffic light.
But why do they have 3 colours?Because there are _three______ __signals______.