单选题 In the flint decades of the twentieth century, the individual gem could not be seen, but could be worked with fruitfully.

【正确答案】 C

【答案解析】blindly盲目地，摸索地；completely十分地，完全地；productively有结果地，有成果地； carefully小心地，谨慎地；fruitfully产量多地，肥沃地。