如图，半圆形光滑轨道固定在水平地面上，半圆的直径与地面垂直，一小物块以速度v从轨道下端滑入轨道，并从轨道上端水平飞出，小物块落地点到轨道下端的距离与轨道半径有关，此距离最大时，对应的轨道半径为（重力加速度为g）（   ）

阅读下列短文，从每题所给的四个选项中选出最佳选项。

     On a hot summer day, Holmes and Dr. Watson went on a camping trip. After a good meal and a bottle of wine, they lay down in a tent for the night and went to sleep.

     Some hours later, Holmes woke up and pushed his friend. “Watson, look! What can you see in the sky?"

     Watson replied, “I see millions and millions of stars.”

      “What does that tell you?” asked Holmes.

     Watson thought for a while. “First, it tells me that we will have a beautiful clear date tomorrow. Second, it tells me that the time is about twelve o'clock. And…”

      “What else?” Holmes asked.

      “Well, it also reminds me that the world is so big and we are so small. What does that tell you?”

     Holmes said with a grin on his face, “You silly guy! It only tells me that someone has stolen our tent!”

读“地球绕日公转示意图”（如图所示），回答：

图中春分点的位置是（  ）