Which best describes a mudflow? sudden movement along a flat, inclined surfacemovement too slow to be observed directlymaterial moving downslope as a thick fluidmaterial falling freely through the air

I just took the quiz and can confirm that the right answer is "material moving downslope as a thick fluid".

Source/Explanation (from my textbook) (keywords that I think best match the right answer have been underlined by me):

Flows are mass movements of material containing a large amount of water, which move downslope as a thick fluid. Flows that move quickly, called mudflows, are common in semiarid mountainous regions, such as parts of southern California.