【正确答案】 C

【答案解析】[解析] 此句可译为“现在的产量是1990年前产量的六倍”。what it is/was是个很有用的句式，表示前面所提到的同类情况，如：Life in the nineteenth century was very different from what it is now. The situation is very different from what it was twenty years ago.