there are many different types of food, some animals spend their lives eating only one type.
A. Since
B. Unless
C. Once
D. While
网友回答
A. Since
B. Unless
C. Once
(答案→)D. While
解析：考查连词：A. Since自从B. Unless除非C. Once一旦， D. While虽然，当…时候，句意：虽然有很多不同类型的食物，一些动物一生只吃一种食物。选D。