小明编写一个类实现设置马的品种、年龄、性别及速度，输出相应的语句，请补全代码。

如下代码输出结果为“一匹阿拉伯12岁的公马，在草原上奔跑的速度为50km/h”。

class  _____① _____       :

def __init__(self, category, gender, age):

self.category = category

_____②_____

self.age = age

self.speed = 0

def get_descriptive(self):

self.info = "一匹" + self.category + _____③_____  + "岁的" +self.gender + "马"

def write_speed(self, new_speed):

self.speed = new_speed

addr = "在草原上奔跑的速度为"

print(        ④        + "，" + addr + str(self.speed) + "km/h。")

horse = Horse("阿拉伯","公",12)

horse.get_descriptive()

horse.write_speed(50)