已知棱长为1的正方体ABCD A 1 B 1 C 1 D 1 中, P,Q是面对角线A 1 C 1 上的两个不同动点. ①存在P,Q两点,使BP DQ; ②存在P,Q两点,使BP,DQ与直线B 1 C45 0 的角; ③若|PQ|=1,则四面体BDPQ的体积一定是定值; ④若|PQ|=1,则四面体BDPQ在该正方体六个面上的正投影的面积的和为定值. 以上命题为真命题的个数是( )
A.
1
B.
2
C.
3
D.
4
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