请教08语法笔记206，(GWD-13-30)

ashcroft

For the first time inthe modern era, non-Hispanic Whites are officially a minority in California, whichamounts to a little less than half the population of the state, down fromnearly three-quarters only a decade ago.
A.    which amounts to a little less than half the population of the state,down from nearly three-quarters only a decade ago
B.    which amounts to a little less than half the population of the state,down from a decade ago, when it was nearly three-quarters
C.    and that amounts to a little less than half the population of the state,down from a decade ago, when they were nearly three-quarters
D.     amounting to a littleless than half the population of the state, down from nearly three-quarters adecade ago    (D)
E.    amounting to a little less than half the population of the state, downfrom what it was a decade ago by nearly three-quarters
关于正确的D选项，解释中说：down from在这里是一个分句～我想问一下，那么这个down算是什么词性？动词？副词？还是介词？
另外有没有哪位NN有类似这种分句形式的例句可以分享一两个，谢谢！：）