What is wrong with these sentences? I am doing some grammar work and have been asked to correct the following， but i'm not sure what is wrong. Can anyone help?

学生们把电台里的中文新闻听懂了。 
不洗澡的坏习惯被小明给改掉了. 
这本书被卖得光.
This is a question regarding sentence structure in Chinese. There are two commonly used sentence structure involved: 把字句 (sentences using the word 把) and 被字句 (sentences using the word 被). These two structures can be freely converted to each other and to sentences containing neither of 把 and 被 (called 陈述句). None of these sentences are inherently wrong, yet many of them can sound unnatural.
For example for the first sentence,
被字句 (Original): 电台里的中文新闻被学生们听懂了。
把字句: 学生们把电台里的中文新闻听懂了。
陈述句: 学生们听懂了电台里的中文新闻。
All three sentences work, but the second and third sound more natural.
Or for the second sentence, 
被字句 (Original): 不洗澡的坏习惯被小明改掉了。
把字句: 小明把不洗澡的坏习惯改掉了。
陈述句: 小明改掉了不洗澡的坏习惯。
The second and third sentence sounds better.
The last sentence is a bit tricky since the person doing the action is not given. We need to add the person doing this action. We may speculate that the book is sold out by a bookstore, so let's use 书店 as the subject.
被字句 (Original)：这本书被卖得光。
把字句: 书店把这本书卖得光。
陈述句:  书店卖光了这本书。
Since we don't know the real subject, it is the best to do with the original sentence.
If you would like more of these type of practices, there are plenty of resources online, many of which are from China and are actually done by Chinese students. An example is this one: https://wenku.baidu.com/view/63b6233f6137ee06eef918b7.html