单项选择题
A.Ⅱ1与Ⅱ2平行 B.Ⅱ1与Ⅱ3垂直 C.Ⅱ2与Ⅱ3垂直 D.Ⅱ3与Ⅱ2平行
A.±31/2/3(i+j+k) B.±21/2/2(i-k) C.31/2/3(i-j+k) D.±21/2/2(i+k)
A.2 B.2×21/2 C.21/2/2 D.1