题干

如图所示,倾角α=30°的固定光滑倾斜杆上套有一个质量为m的圆环,圆环与竖直放置的轻质弹簧上端相连,弹簧的下端固定在水平地面上的A点,开始弹簧恰好处于原长h,现让圆环由静止沿杆滑下,滑到杆的底端(未触及地面)时速度恰好为零,已知当地的重力加速度大小为g.则在圆环下滑的整个过程中(  )

A:圆环与地球组成的系统机械能守恒

B:弹簧的弹性势能一直增大

C:弹簧的弹性势能增大了mgh

D:弹簧的弹性势能最大时圆环动能最大

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C

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Discovering Tasmania

    The island of Tasmania is separated from mainland Australia by the Bass Strait. The island is a place of natural beauty and has more than 2,000 km of walking tracks and 18 national parks. If you go on a tour, you'll discover a wild and beautiful place where the people are friendly and the food is delicious. If you don't like walking, there are other tours you can choose from including a river cruise and cycling. You can also combine your tour with fishing, sailing or sunbathing on the beach. 

    One of the most incredible places to walk is along the Tarkine coast which is located in the north-west of Tasmania. It's such a wild and remote area that you can easily complete your walk without seeing anyone apart from the members of your group and your two guides. The area contains the largest temperate rainforest in Australia which is home to more than 50 endangered species. It is also home to many Aboriginal Heritage Sites. Your guides will provide you with plenty of information about the area as you complete that part of your tour. During your tour, you'll come across rivers, mountain ranges, spectacular waterfalls, wildlife and long wild beaches. It will be an experience you won't easily forget. 

Tour Itinerary:

Day 1:

    You're picked up from your hotel in the town of Launceston and driven to the Tarkine. You then complete a three-hour walk through the forest before arriving at your camp at Mystery Creek. There you will enjoy a delicious meal cooked by your guides.

Day 2:

    After breakfast, you continue deeper into the rainforest, passing some of the tallest trees in the world as you go, and stopping for lunch and then camp in the evening.

Day 3:

    The highlight of today's hike is the Tarkine Falls, a beautiful 15-metre waterfall.

Day 4:

    Today you can stay at the camp and bathe in the Tarkine Falls, or you can go for a day hike for more fantastic views of the forest.

Day 5:

    After a last hike through the forest, you are picked up at about 4:00 p.m. and you arrive in Launceston at around 7:00 p.m.

    The tour includes two professional guides, transport to and from the rainforest, all food while on the tours and all safety equipment. You should buy or hire recommended camping equipment including: backpacks, sleeping bags, sleep mats, head torches, rain coats and trousers.