The population of this fatory was reduced __ 3,000 cars a years, while costs were increased __ 3.5 percent. The manager throught it necessary to tahe action.A: to/to B: to/byC: by/byD: by/to该选什么?还有为什么?请给我一详细点的解释~谢谢~谢谢你们,我知道答案了~