these coins are of different shapes sizes and colors they all date back to a long long

问题补充：

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

答案：

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。