I'll take option C)
What we are seeing is neither a 'true' darkening or clouds or smoke.
It's a camera-glare / gamma / contrast adjustment effect due to the setting sun coming into view of the camera. As the sun moves towards the center of of the frame, the gamma level of the overral frame decreases to accomodate the extra light and renders the sun visible.
In theory it ought to be darkest around the sun itself, but
The One True Darkening of the sky will really begin in about 8-12 newpix (as a new arc with the sun at the center this time) when the sun dips below the horizon.
At this point we'll start seeing stars (also moving in the same direction as the sun of course). (It will be interesting to see if any of the stars line up with actual constellations).
After a further 10 newpix or so, the image will settle, with an ambient sky brightness probably around the hex 40 level (thats 64 in decimal or 25%).
Also a molpy or two will make a quick appearance -- quite possibly the molpycat as previously mentioned then it will jump back to Cueball with no further fading.
I therefore predict we will jump to Cueball / Megan at newpix 2388 give or take 3 newpix.
So yeah, C) Final answer.
Edit: Ooh cool top of the paginess. I decree everyone have a good time on this page.