A:whom
B:who
C:which
D:where
— There _______ a good progamme called Across the Sea to See You (漂洋过海来看你) on Zhejiang Satellite TV.
(I)求证:AA1⊥BC;
(II)把四棱锥A1﹣BCC1B1绕直线BC旋转一个角到A′﹣BB′C′C,使平面ABC与BB′C′C重合,求该旋转角的余弦值.