Задайте общий, специальный, разделительный вопросы и вопрос к подлежащему к этим

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?(удивительно звучит,но....это верно)

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?