because people speak English
The alphabet comprises 26 letters.
The alphabet is composed of 26 letters.
The alphabet consists of 26 letters.
The alphabet is constituted by 26 letters.
Which is better?
Or do you discriminate those depending on circumstances?
If there are any other better or similar expressions, would you mind if you would tell me?
Thank you for the comment.
>The sentence must be passive
This depends on a verb you choose, I think.
For example, you can see the example sentence below in some dictionary.
-The Soviet Union comprised several socialist republics.http://dictionary.reference.com/browse/comprise
You are very welcome. You are right all verbs shouldn't be passive ,however, I am talking about the sentences you wrote. "The alphabet" needs a passive sentence since we don't have a subject.