By saying “Middle children are invisible” in paragraph 3, Lewis means that middle children 。

B
文章第三段提到 Lewis found that in families with three or four children, dinner conversation is likely to center on the oldest child, who has the most to talk about, and the youngest, who needs the most attention. 可以看出老大和老小都是谈话的焦点，只有中间的孩子是被人忽视的。