—The Summer Palace is wonderful. Have you ever visited any other interesting places?

—Yes. Also, we _______ to the Great Wall. [ ]

A. have gone

B. have been

C. had gone

D. had been

某研究单位把蔬菜大棚中空气中CO₂浓度提高3﹣5倍，让O₂的浓度尽量减少（或用其他方法收集起来），结果使农作物的葡萄糖产量大大提高．下图为光合作用的方程式：

试依据光合作用的方程式分析解释这种现象 ____                                                  _____ ．