一个圆锥摆的摆线长为<img src="/lqwvjeImg/6e/6ec9f7968f6ac606a8cc88102449b1d3.png" style="width:10px;height:18px;" >,摆线与竖直方向的夹角恒为<img src="/lqwvjeImg/c5/c54a168c48078ea085de13ac82846100.png" style="width:14px;height:18px;" >,如图所示。则摆锤转动的周期为( )<img src="/lqwvjeImg/77/77f8c174349fa74b8b13385972ddf6e6.png" style="font-size: 20px;" ><br>· <img src="/lqwvjeImg/3f/3f3ca9ee3ebd3756e255da8194fa9cff.png"style="width:29px;height:49px;"> .<br>· <img src="/lqwvjeImg/fb/fba841b56817f5ebc4b04110a4bb42b5.png"style="width:58px;height:49px;">.<br>· <img src="/lqwvjeImg/09/09f35c8fa2d604a56a9bdec9c0056751.png"style="width:48px;height:49px;">.<br>· <img src="/lqwvjeImg/3b/3b9c6dc2a28386fae9850e10f615eeb4.png"style="width:77px;height:49px;">