Superconductive materials are usually most effective

at very low temperatures
at very high temperatures
when a magnetic field is applied
under high pressures

Which of the following may one conclude from a map that shows the average ph value of rainfall in the u. s.? acid rain is a more serious problem on the east coast. acid rain falls equivalently across the continental u. s. there are more factories on the west coast. the midwest has fewer forests than the rest of the u. s.