动词 is 是系词 （英语叫 linking verb）,
系词的规则是要和前面的名词一致,因此在有些情况下可接复数单词,
比如：
Joe's favorite dessert is blueberry muffins.
(句子和 Blueberry muffins are Joe's favorite dessert.一样)
Beijing's biggest traffic problem is cars that do not obey traffic rules.
(= Cars that do not obey traffic rules are Beijing's biggest traffic problem.)