题目

小肠绒毛壁、毛细血管壁、毛细淋巴管壁都很薄，仅由一层上皮细胞构成。（  ） 答案：√ Playing QQ’s “Happy Farm” by planting, watering, fertilizing, spraying, harvesting and selling of virtual vegetables, fruits and flowers, we can find out _____ all the excitement is about. A. that                    B. if                   C. what                    D. when