Tom was thinking about how to explain to the teacher why he was late for class, when a good idea _______him.

A．happened

B．brought

C．occurred

D．struck

如图所示，物体A用轻质细绳与圆环B连接，圆环B套在固定竖直杆MN上，一水平力F作用在绳上的O点，整个装置处于静止状态。现将O点缓慢向左移动，使细绳与竖直方向的夹角增大。下列说法正确的是（）

A.水平力F逐渐增大

B.O点能到达与圆环B等高处

C.杆对圆环B的摩擦力增大

D.杆对圆环B的弹力不变