题目内容：
如图， 在三棱柱 ABC﹣ A 1 B 1 C 1 中， 点 B 1 在底面 ABC 内的射影恰好是点 C， 点D 是 AC 的中点， 且 DA＝DB．
（1） 证明： AB⊥CC 1 ．
选择题：如图， 在三棱柱 ABC﹣ A 1 B 1 C 1 中， 点 B 1 在底面 ABC 内的射影恰好是点 C， 点D 是 AC 的中点， 且 DA＝DB．（1）
- 题目分类：高考
- 题目类型：选择题
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A.充要条件
A.充要条件
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