斜三棱柱ABC﹣A₁B₁C₁中，AA₁=AC=BC=2，∠A₁AC=∠C₁CB=60°，且平面ACC₁A₁⊥平面BCC₁B₁，则A₁B的长度为____．

在日常生活中,法律如同空气和水一样,时刻与我们相伴。这一浅显的说法告诉我们一个深刻的道理是(　　)

用所给动词的正确形式填空。

I____(not know) him two years ago.

When it is fine tomorrow, we____ (go) to the park.

Peter____ (not go) to school yesterday because he ____(be) ill.

When Linda ____ (be) young, she danced very well.