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

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?

While the numerical aperture can be used to compare resolutions of various objectives, it does not indicate how far the lens could be from the specimen.
While the numerical aperture can be used to compare resolutions of various objectives, it does not indicate how far the lens could be from the what?

Group 15 of the periodic table is also called nitrogen group.

Group 15 of the periodic table is also called which group?