clarification needed : Computed Style Value of 'font-family'

Let's suppose that the following CSS style declaration applies to a 
given element :

  font-family : Helvetica, Arial, sans-serif

What is the computed style value for 'font-family' on this element, 
supposing that Helvetica font is not available on the system but Arial is ?

(a) "Helvetica, Arial, sans-serif"
(b) "Arial"
(c) "Arial, sans-serif

????? I can't find any clue about it in DOM Level 2 or CSS 2 specs.

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