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

A student should use a balance to compare the masses of two small rocks.
Which tool should a student use to compare the masses of two small rocks?

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

Synostoses unite the sacral vertebrae that fuse together to form the adult sacrum.
Synostoses unite the sacral vertebrae that fuse together to form the what?