图为①②③三地夏至日一天内直立杆的影子朝向和长度变化示意图，杆的长度均为1m。读图，①②③三地纬度由高到低的顺序是(    )

Dear Lucy,
   We are back in Garden City now. We had    1   good trip.
   I    2  to collect my photos yesterday. They    3great! I showed these photos    4  my classmates. They all   5  them.
   There is a photo of Ben and Simon  6   the Great Wall. There is one of you and  7in the Forbidden City(紫禁城).
We really enjoyed    8vacation. We were sad(悲伤)to   9   you.
   You must   10and visit us in Garden City at Chinese New Year.
                                                                                                                                                          Love,
                                                                                                                                                          Kitty

记听课笔记是历史学习的好习惯。小张在笔记本中记录了以下关键词：城门立木取信、建立县制、奖励军功等。由此判断，他学习的主要内容是（  ）