影响旅游安全的因素主要包括(　　)
①自然灾害     　②旅游硬件设施情况　③社会文化环境　 ④旅游地的经济水平

静止在水平面上的A、B两个物体通过一根拉直的轻绳相连，如图，轻绳长L=1m，承受的最大拉力为8N，A的质量m₁=2kg，B的质量m₂=8kg，A、B与水平面间的动摩擦因数μ=0.2，现用一逐渐增大的水平力F作用在B上，使A、B向右运动，当F增大到某一值时，轻绳刚好被拉断（g=10m/s²）．

    2014 is the year of horse in China. Do you know anything about the horse?

    The horse is strong and runs very fast. He's a useful and hard-working animal. The horse learns to know his owner soon. He loves his owner, even after his owner loses his life. Here's a story about such a horse. In a war, the owner passed away(死), and fell from his horse. When people found his body some days later, the horse was still standing beside it. During that long time, the horse didn't leave the body of his owner, without food or water, in the coldest weather, driving away the birds that might eat it. Wasn't he a great animal?

下列各句中，没有语病的一项是（    ）