Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

Filter feeders called rotifers collect digestive and excretory wastes in cloacal bladder primitive organ.
Filter feeders called rotifers collect digestive and excretory wastes in what primitive organ?

Gravity makes soil and rock particles move gradually down the slope during each thaw.
What makes soil and rock particles move gradually down the slope during each thaw?

Terminal speed is another name for critical speed.

What is another name for critical speed?