题目

In 1924, Hachiko was brought to Tokyo by his owner, Hidesaburo Ueno, a professor in the agriculture department at the University of Tokyo. During his owner's life Hachiko saw him off from the front door and greeted him at the end of the day at the nearby Shibuya Station. The pair continued their daily routine until May 1925 when Professor Ueno didn't return on the usual train one evening. The professor had suffered a stroke at the university that day. He died and never returned to the station where his friend was waiting. Hachiko was given away after his master's death but he routinely  如图所示，一个质量为m的小物体静止在固定的、半径为R的半圆形槽内，距最低点高为R/2处，则它受到的摩擦力大小为                  （    ）     A．       B．     C．      D．