Many people keep dogs as pets because dogs are regarded as () friends to people.

A. lonely

B. only

C. loyal

D. fair

如图所示，天花板上固定一根细绳，绳系一小球，则下列说法中正确的是（　　）

A．小球对绳的拉力使小球发生形变．

B．绳子形变后对小球产生拉力

C．小球对绳的拉力的方向竖直向上

D．小球对绳拉力就是绳对天花板的拉力