A、1/n
B、1/<img src="http://admintk.100xuexi.com/UpLoadImage/2013-05-07/0045b0f0-b663-4393-aaec-dc695ad479e9.jpg" width="18" height="27" />
C、<img src="http://admintk.100xuexi.com/UpLoadImage/2013-05-07/4aa4d57a-a681-4e05-b51b-1179be7f6b1f.jpg" width="38" height="41" />
D、<img src="http://admintk.100xuexi.com/UpLoadImage/2013-05-07/79f1b648-c22f-40d1-8874-f443e84abde4.jpg" width="36" height="34" />
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