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

Ex Input:
Simple carbohydrates can be classified in terms of chain length.

Ex Output:
Which carbohydrates can be classified in terms of chain length?


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:
Mercury is closest to the sun.

Ex Output:
Which planet is closest to the sun?