I often see some old people ( )in the park on my way to school .(exercise)

exercise原型
people是复数为什么不用exercisingoften是一般现在时的标志例如 She often spends hours dressing her hair.她经常花数小时整理头发My father often tells me that knowledge is power.我父亲经常告诉我说知识就是力量。而exercising是表示一直在运动 或者表主动 但是这儿不是