Shorter answers

Hi everyone!
Can we make the answers to these questions shorter ?
1.How could the police avoid mistakes when using radar guns?
The police could avoid mistakes if the radar guns are used correctly and police officers are traibed to use them properly.
Is it correct to say:
-If the radar guns are used correctly and police officers are trained to use them properly.(omitting the main clause) if that's correct, can we make the sentence more shorter?
2.what does the whole section of books about different careers in a library describe?
-It describes the various jobs and tells what skills or qualifications we need to do them. (Can we make the answer shorter?)
3.When it was invented?
During the World War 2.(Instead of saying It was invented during.....
Thanks in advance