阅读短文，选择正确答案。
    We are students of Yuying Primary School(小学). In our school, there are many teachers and students. There are twenty­six girls and twenty­four boys in our class. Our classroom is big. There are six windows and two doors. The walls(墙) are white. There are two pictures on the wall. A big blackboard is in the front of the classroom. Our desks and chairs are new. Our classroom is very nice. I like it very much.

如图，在⊙O的内接四边形ACDB中，AB为直径，AC：BC=1：2，点D为弧AB的中点，BE⊥CD垂足为E．

设向量 $\overrightarrow{a}$ ， $\overrightarrow{b}$ 是相互垂直的单位向量，向量λ $\overrightarrow{a}$ + $\overrightarrow{b}$ 与 $\overrightarrow{a}$ ﹣2 $\overrightarrow{b}$ 垂直，则实数λ=____．

确立人大代表候选人的选举方式（    ）
①属于差额选举    ②有助于选民了解候选人，提高选举积极性
③属于等额选举    ④有利于比较充分地考虑当选者结构的合理性

四倍体西瓜植株与二倍体西瓜杂交，结出的四倍体西瓜中有250粒种子，形成
这250粒种子的过程中发生受精作用的精子数目是