如图所示是一根重为G的均匀木棒OA，能以支点0自由转动，现用力F_C作用于木棒的A端，使木棒OA在水平位置上保持平衡。当力F由FC方向逐渐转到F_B时，力的大小变化为 (      )
                                                                                                                                                                                                                                 

A．一直在减小

B．保持不变

C．先减小后增大

D．先增大后减小