昨天的考题 用they 还是 it 指代 sums of capital ?

昨天一朋友语法题，她说真郁闷！  想了3分钟还是在这个最后一个疑点上卡住了。

我记得OG有一题：

29.   In three centuries—from 1050 to 1350—several million tons of stone were quarried in France （for the building of eighty cathedrals, five hundred large churches, and some）  tens of thousands of parish churches.

其中用法是： tons of stone were... 用的是were, 所以似乎tons of stone 表示复数，  所以sums of capital 也表示复数，应该用 they 指代？

NNs？where are you?!

sums of + 可数名词和不可数名词作主语，动词都用复数

Hope this could help