7. Which statement about the use of independent variables in controlled experiments is correct?
a A different independent variable must be used each time an experiment
is repeated.
b. The independent variable must involve time,
c. Only one independent variable is used for each experiment.
d. The Independent variables state the problem being tested

The answer is atmosphere. there are four spheres of the earth: 1. geosphere (from greek word 'geo' = ground), 2. hydrosphere (from greek word 'hydro' = water), 3. biosphere (from greek word 'bio' = life), and 4. atmosphere (from greek word 'atmo' = air). a magnetosphere is a region of space around astronomical objects controlled by the magnetic field of that object. through the process of elimination, since lightning is present in the air, it can be found in the atmosphere.