A:re
B:ee
①数量多 ②肺泡壁很薄,仅由单层细胞构成 ③肺泡外表面缠绕着丰富的毛细血管和弹性纤维
59 - 18 = 13 + 23 = 0.25+ 74 = 112 - 38 - 58 =
43 + 12 = 1- 35 = 0.875- 12 = 437 - 37 + 18 =
17 + 16 = 12 - 15 = 56 - 29 = 12 + 115 =
设a∈R,则“a=1”是“直线l1;ax+2y=0与直线l2;x+a+1y+4=0平行的( )