题目

阅读下列短文，从每题所给的（A、B、C、D） 四个选项中选出最佳选项。Sleep apnea is when you have pauses in breathing while you're asleep.These pauses can last from seconds to minutes. You may have difficulty breathinga few times or dozensof times an hour. These breathingpauses can be dangerous if they cause the oxygen level in your body todrop or disturb your sleep. When oxygen drops, your brain does whatever it can to get you to resume breathing. And then you maysnore (打鼾),gasp, or make a choking sound.Sleepapnea is a common disorder. Anyone can develop it.Sleep（10分）如图所示，斜劈放在粗糙的水平地面上，它的斜面是光滑的，斜面长为L，倾角为θ，质量为m的小物体由斜面顶端从静止下滑至底端，而斜劈与地面保持相对静止，求（1）物体下滑的加速度？（2）斜劈受到水平地面的摩擦力大小和方向？