答案：C

题目分析： 与现在事实相反：条件从句用动词过去式，主句用would/could/should/might+动词原形；与过去事实相反：条件从句用had done，主句用would/could/should/might+have done；与将来事实相反：条件从句用动词过去式/should+动词原形/were to do，主句用would/could/should/might+动词原形。句意：如果我是你，我上课就会保持安静，认真听课。故选C。