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

2 hydrogen, 1 oxygen atoms combine to make up a molecule of water.
What atoms combine to make up a molecule of water?

Gardasil is the vaccine to prevent infection of hpv.
What is the vaccine to prevent infection of hpv?

The length of the sloped surface of a ramp is referred to as input distance.
The length of the sloped surface of a ramp is referred to as what?