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

Virtually all aquatic organisms depend directly or indirectly on protists for food.
Virtually all aquatic organisms depend directly or indirectly on what for food?

Interstitial fluid are all hormones secreted into before entering the circulatory system.
Where are all hormones secreted into before entering the circulatory system?

The frequency of a wave is the reciprocal of the period of the wave.

The frequency of a wave is the reciprocal of what other attribute of the wave?