Sorry, but your question isn't simple at all!
>I would like to determine whether a element C with a selected attribute 'car' is the first element within <A>
Do you mean
(1) "is the first child element of <A>"
(2) "is the first child element of <A> whose name is <C>"
(3) "is the first child element of <A> that has a selected attribute whose name is 'name' and whose value is 'car'
(4) "is the first descendant element of <A>"
(5) "is the first descendant element of <A> whose name is <C>"
(6) something else....?
>something like this Structure can appear in font of this
What does "this Structure" refer to?
(1) The whole of the XML fragment you have shown?
(2) The <B> element?
(3) Something else?
>In other way there is no element which satisfies ancestor::A and preceding::C
In your example, that condition is satisfied by the first C, and is not satisfied by the second C. But until now, I thought your rules would match the second C but not the first. Please be more clear!
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference