Historians believe that, for most countries involved in the conflict, world war i's root causes were varied and complex, though many still point to the assassination of archduke franz ferdinand as the immediate precursor to war.
(a) historians believe that, for most countries involved in the conflict, world war i's root causes were varied and complex
(b) historians believe that variation and complexity were there for most countries involved in the conflict by the world war i's root causes
(c) the historians' belief is, for most countries involved in the conflict, root causes were varied and complex for world war i
(d) for most countries involved in the conflict, the historians' belief that world war i's root causes were varied and complex
(e) for most countries being involved in the conflict, historians believe that the root causes that world war i had were varied and complex