GMAT Sentence Correction (SC) | OG 2016 - Discussion of greenhouse effects have usually had as a focus

Hi experts,

1/
(B) Discussion of greenhouse effects has usually had as its focus whether Earth would get warmer and what the extent would be,

as I know, the correct is focus on if focus works as a verb,
I wonder whether prep "on" should follow immediately if "focus" works as a noun, that's why I cross off B, but I am not sure whether the reason is valid, because OE does not mention it.

please conform,

2/
(C) Discussion of greenhouse effects has usually focused on whether
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