问题补充:
there are many different types of food, some animals spend their lives eating only one type.
A. Since
B. Unless
C. Once
D. While
答案:
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。