— Is your sister's hair long now?
— Yes, ____. ( )
A:it is
B:she is
C:it was
古诗文默写填空① , 把酒话桑麻 。② , 娇儿恶卧踏里裂。③ , 俶尔远逝。④自经丧乱少睡眠, 。⑤ 。人不寐,将军白发征夫泪。⑥《过故人庄》中描绘优美宁静的田园风光的诗句⑦《浣溪沙》中体现作者积极的人生态度、豁达的胸襟的诗句
(Ⅰ)求证:AC1⊥A1B;
(Ⅱ)求直线AB与平面A1BC所成角的正切值.