Files and sizes

Alejandro wants to explain the importance of file extensions to his friend.

Which of these explanations is the best?

(Choice A)
When a file has an extension, users can open files in applications they don't own.
(Choice B)
When a file has an extension, the computer can decide what application to open it in.
(Choice C)
When a file has an extension, the computer is allowed to store additional metadata about it.
(Choice D)
When a file has an extension, the user knows who created the file.
(Choice E)
When a file has an extension, the computer knows exactly how much memory it needs to allocate for it.

Computers and the Internet