— _____ will your father return home? It's a long time since I saw him last time.
— Just in two days.
若向量 a→ , b→ 满足:| a→ |=1,( a→ + b→ )⊥ a→ ,(3 a→ + b→ )⊥ b→ ,则| b→ |=( )
《龟兔赛跑》的故事里,小乌龟取得胜利的原因是( )。