What needs to be emphasized in this step when reading?
"Turn the two-bottle assembly over, and place the assembly on a table with the filled bottle on top. Watch the water slowly drip down into the lower bottle Now, rapidly rotate the bottles in a horizontal circle a few times. Observe the formation of a funnel-shaped vortex..."

a.
rapidly rotate the bottles in a horizontal circle
c.
both A and B
b.
Watch the water
d.
neither A nor B