American ingenuity and science are constantly ________ improving

American ingenuity and science are constantly ________ improving the quality of products.
A、for work
B、with work
C、at work
D、in work
【正确答案】：C
【题目解析】：be at work起作用
句意：美国人的聪明才智和科学不断提高产品的质量。