填空题

Unlike most plants, animals are living beings that have the ability to move (11) in search of food. Scientists divide animals into two groups. The animals in the first group are those with backbones (12) the second group includes those without (13) . Birds, fish and mammals belong to the first group. Among them, animals are (14) the most complex animals. Mammals have (15) or fur. They nurse their young. (16) people use the word "animal" when they are talking about mammals such as the dog, cat, cow, horse, rabbit or (17) . Most animals of the second group (18) in water because the pull of gravity makes it difficult for them to (19) about on land. The largest animal on earth is the blue whale. A whale often grows more than 30 meters (20) and weighs more than 100 tons. The elephant is the largest land animal and the ostrich is the largest bird.