The letter _____ three days ago and it _____ yesterday.
A:had post; had arrived
B:was posted; arrived
C:posted; arrived
D:had been posted; was arrived
肺泡内的气体进入血液需要经过( )
—All right.
主族
周期
ⅠA
ⅡA
ⅢA
ⅣA
ⅤA
ⅥA
ⅦA
0族
2
①
②
③
3
④
⑤
⑥
⑦
⑧
4
⑨
(Ⅰ)求椭圆E的方程;
(Ⅱ)判断▱ABCD能否为菱形,并说明理由.
(Ⅲ)当▱ABCD的面积取到最大值时,判断▱ABCD的形状,并求出其最大值.