Why isn't this sentence in a passive form? [closed]

Question

I found this sentence in a grammar book for grade 10

Which CD sells the most? A traditional music CD.

I wondered why it isn't in a passive form, or just because it's used in spoken context?

Answer 1

There's a common feature in English known as the "ergative construction", "middle construction", or "labile construction", though it's not quite the same as an actual ergative case (as found in Basque) or middle voice (as found in Ancient Greek).

In this construction, a verb that normally takes two nouns (Alice broke the window, Bob sold the CDs, Claire swung the door open) takes only a single noun, but that single noun was the object in the two-noun construction, not the subject (the window broke apart, the CDs sold well, the door swung open).

When this happens, unlike with the passive, the verb form doesn't change at all, and there's no way of including the original subject (the window was broken by Alice, *the window broke apart by Alice). And this only works with certain verbs: Dave ate the soup, *the soup ate. (The star means "this isn't valid".) The verbs that do allow this are conventionally called "ergative verbs", "middle verbs", or "labile verbs".

Answer 2

The word "sells" here in the English language of today has a different meaning from "is sold", and Draconis' answer does not apply here (even if it may have historically contributed to the word's meaning). The following have very different meanings:

This book is sold.

This book sells.

The first means that the book in the current context (e.g. the one I was just talking about) is now sold (i.e. the sale has been done). The second means that the book title (not an individual book) is easy to sell (or popular), because here "sells" means something like "goes in a sale" and "easily" is implied if not explicitly or otherwise stated. One can make explicitly how well something sells using an adverb like this:

This book sells/sold well/quickly/slowly/poorly.

This book sells like hot cakes.

It would be incorrect to use the ordinary "sell" for this:

(WRONG) This book is sold well/poorly.

(WRONG) This book is sold like hot cakes.