She felt like giving him an even harder kick , but her horse stayed stubbornly a few strides behind his no matter how she slapped its sides . " Do n't tell me to shut up ! You know what you said ! " He rolled his shoulders .

Question: What is the reason her horse would have stayed stubbornly behind his horse ?
Answer: None of the above choices .Well trained horses know not to interfere with another horse / rider and maintain a sociable distance .The horse was trying to mount the mare in front .The horse was being beaten by the rider in front of them .