A channel consisting of one or more independent producers, wholesalers, or retailers that seek to maximize their own profits even at the expense of profits for the channel as a whole is a(n) ________.
【简答题】已知平行六面体ABCD-A 1 B 1 C 1 D 1 ,AC 1 与平面A 1 BD,CB 1 D 1 交于E,F两点.给出以下命题,其中真命题有______(写出所有正确命题的序号) ①点E,F为线段AC 1 的两个三等分点; ② ED 1 =- 2 3 DC + 1 3 AD + 1 3 AA 1 ; ③设A 1 D 1 中点为M,CD的中点为N,则直线MN与面A 1 DB有一个交点; ④E...