马场有甲、乙、丙、丁四个口可以提供人进入游玩，如果现在开了甲、乙两个口，经过了4.5小时游客全部能够进入，如果开乙、丙两个口，游客3小时能全部进入，如果开丙、丁两个口，游客3.5小时全部进入。若只打开甲、丁两个口，则需要几个小时，游客能全部进入？（ ）
A、<img width="23" height="41" src="http://admintk.100xuexi.com/UpLoadImage/2016-08-23/59bb6a25-1d19-4068-86bf-74dec5edb3e0.png" />
B、<img width="23" height="41" src="http://admintk.100xuexi.com/UpLoadImage/2016-08-23/b1d4c160-5554-472f-993d-14fe6007aae4.png" />
C、<img width="29" height="41" src="http://admintk.100xuexi.com/UpLoadImage/2016-08-23/2303c5d4-0675-4837-a114-c32ebf97b308.png" />
D、<img width="29" height="41" src="http://admintk.100xuexi.com/UpLoadImage/2016-08-23/089f9dd7-bf05-44ca-869e-998eebaf52d4.png" />
A、<img width="23" height="41" src="http://admintk.100xuexi.com/UpLoadImage/2016-08-19/0faecb30-9a03-4b3d-aee9-bbe1ed8f6480.png" />
B、<img width="23" height="41" src="http://admintk.100xuexi.com/UpLoadImage/2016-08-19/2e9e55df-64dc-4d2e-af1c-8676b85f2a4c.png" />
C、<img width="29" height="41" src="http://admintk.100xuexi.com/UpLoadImage/2016-08-19/42b6ee92-a4de-4855-bf0f-371a0876c4df.png" />
D、<img width="29" height="41" src="http://admintk.100xuexi.com/UpLoadImage/2016-08-19/a09725a5-78a0-416f-823f-cc27101bad2e.png" />
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