阅读下列短文，从每题所给的四个选项 （A 、B 、C 和 D ）中 ，选出最佳选项。
Anne LaBastille was born in New York City and grew up in New Jersey. Her first experience with the wilderness was in the Adirondacks in the northeast of New York, where she worked at a summer resort (度假胜地) to earn money for college tuition (学费) by caring for the horses, giving riding lessons, and working as a waitress. And she had many chances to begin her adventure in the Adirondack wilderness.
Anne returned to school in the fall, but she continued to spend as much time as she could in the Adirondacks. She grew to love her time alone in the mountains. Anne graduated from college with a bachelor's degree in conservation of natural resources (资源) and began working for the National Audubon Society in Florida as a wildlife tour leader.
Although Anne took great pleasure in showing people the animals living in t he Florida Keys and the Everglades National Park, she longed for the mountains in the northern parts of New York. Finally she decided to build a cabin near Black Bear Lake. Over the years, however, more and more tourists began hiking near her cabin. As a result, Anne decided to build another cabin deeper in the woods at Lily Pad Lake.
Anne lived in the woods for most of her life. She enjoyed living alone in the woods, and her life was far too busy for her to be lonely. When she was not writing books, she wrote articles for National Geographic, Reader's Digest, and other magazines. In her later years, Anne conducted research in Guatemala on an endangered bird called the grebe. She also lectured nationwide about ecology. Besides, Anne worked with a number of organizations dedicated to (致力于) conservation.
As a respected guide, author, and conservationist, Anne not only loved the land but also had found a way to become part of it.
时刻导线框的上边恰好与磁场的下边界重合(图中位置I)，导线框的速度为
经历一段时间后，当导线框的下边恰好与磁场的上边界重合时(图中位置II)，导线框的速度刚好为零。此后，导线框下落，经过一段时间回到初始位置I。则
