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There are about ten major vitamins.
They are usually named by a letter of the alphabet.

There are about ten major vitamins.

Are there about ten major vitamins?

How many major vitamins are there?

There are about ten major vitamins, aren't there?

What is there?(странно звучит,но....это правильно)

They are usually named by a letter of the alphabet.

Are they usually named by a letter of the alphabet?

How are they usually named?

They are usually named by a letter of the alphabet, aren't they?

What is named by a letter of the alphabet?