A、<img width="24" height="40" src="http://admintk.100xuexi.com/UpLoadImage/2013-05-07/4b4553c3-a355-4433-a861-0fad513ed93f.png" />
B、<img width="24" height="40" src="http://admintk.100xuexi.com/UpLoadImage/2013-05-07/2c5ef9dd-4af9-4a59-bee6-79c264758e35.png" />
C、<img width="24" height="39" src="http://admintk.100xuexi.com/UpLoadImage/2013-05-07/acb8460e-fac5-4188-a91f-a4c13ae249c8.png" />
D、<img width="24" height="40" src="http://admintk.100xuexi.com/UpLoadImage/2013-05-07/bb047f70-e2f5-41a1-8393-9978c041d197.png" />
查看答案
，单位向量
，则
=（ ）。