Which of the following would be an example of an independent variable?

the options are
1.rainfall
2.population
3.year
4.income

Determine the age of a fossil if it had only one eighth of its original carbon-14 content remaining?

Asolid cylinder and a cylindrical shell have the same mass, same radius, and turn on frictionless, horizontal axles. the cylindrical shell has light-weight spokes connecting the shell to the axle. a rope is wrapped around each cylinder and tied to blocks of equal masses that are held the same height above the ground. both blocks are released simultaneously. the ropes do not slip. which block hits the ground first? or is it a tie?

Could we see a galaxy that is 20 billion light-years away?