動(dòng)詞 is 是系詞 （英語叫 linking verb）,
系詞的規(guī)則是要和前面的名詞一致,因此在有些情況下可接復(fù)數(shù)單詞,
比如：
Joe's favorite dessert is blueberry muffins.
(句子和 Blueberry muffins are Joe's favorite dessert.一樣)
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.)