Most fishes and many reptiles have ribs along most of the spine, but in mammals ________ only in the chest area.
句意： 大多数鱼类和爬行动物的脊椎大部分都有肋骨， 但哺乳动物的肋骨只分布在胸部。 缺乏主语和谓语， 而只有A项具备完整的主谓关系。