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

Ex Input:
The active transport of ions across the membrane causes an electrical gradient to build up across the plasma membrane.

Ex Output:
The active transport of ions across the membrane causes an electrical gradient to build up across the what?


Ex Input:
Water plus other substances makes a(n) solution.

Ex Output:
Water plus other substances makes what?


Ex Input:
Bacteria are the most numerous species on the planet.

Ex Output:

What is the most numerous species on the planet?