若复数2+i1-ia∈R是纯虚数(i是虚数单位),则a的值为 ( )
In France you are not supposed to eat anything with your hand e____ bread.
ATP是细胞内的能源物质,医疗上用ATP注射液可治疗心肌炎。若人体静脉滴注ATP药物,ATP到达心肌细胞内最少要穿过多少层细胞膜 ( )