【简答题】Language learning begins with listening. Individual children vary greatly in the mount of listening they do after 【S1】______ they start speaking, and late starters are often long listeners. Most child...
【简答题】如图,正方体ABCD-A 1 B 1 C 1 D 1 棱长为1,点M∈AB 1 ,N∈BC 1 ,且AM=BN≠ 2 ,有以下四个结论: ①AA 1 ⊥MN,②A 1 C 1 ∥ MN;③MN ∥ 平面A 1 B 1 C 1 D 1 ;④MN与A 1 C 1 是异面直线.其中正确结论的序号是______(注:把你认为正确命题的序号都填上)