下面是你要写的作业，这段向你介绍了“if语句”。把这段输入进去，让它能正确执行。然后我们看看你是否有所收获。

people = 20
cats = 30
dogs = 15


if people < cats:
    print "Too many cats! The world is doomed!"

if people > cats:
    print "Not many cats! The world is saved!"

if people < dogs:
    print "The world is drooled on!"

if people > dogs:
    print "The world is dry!"


dogs += 5

if people >= dogs:
    print "People are greater than or equal to dogs."

if people <= dogs:
    print "People are less than or equal to dogs."


if people == dogs:
    print "People are dogs."
你应该看到的结果
$ python ex29.py 
Too many cats! The world is doomed!
The world is dry!
People are greater than or equal to dogs.
People are less than or equal to dogs.
People are dogs.
$

加分习题
猜猜“if语句”是什么，它有什么用处。在做下一道习题前，试着用自己的话回答下面的问题:

你认为 if 对于它下一行的代码做了什么？
为什么 if 语句的下一行需要 4 个空格的缩进？
如果不缩进，会发生什么事情？
把习题 27 中的其它布尔表达式放到if语句中会不会也可以运行呢？试一下。
如果把变量 people, cats, 和 dogs 的初始值改掉，会发生什么事情

来源：https://www.jb51.net/shouce/Pythonbbf/latest/ex29.html