these coins are of different shapes, sizes and colors, they all date back to a long, long time ago.
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，既然B. Unless除非C. Once一旦D. While当…时候，虽然，句意是：虽然这些硬币形状颜色大小不同，但它们都可追溯到很远很远以前。选D。