What can be increased in order to increase the magnetic field?

the size of the iron core

the magnetic poles

the wire resistance

the current

The freezing and boiling point of a substance changes as the air pressure around it changes. for example, at a lower air pressure (higher altitude) it is easier for water molecules to escape from liquid into the air. in a high altitude city such as denver, colorado compared to a sea-level city such as houston, texas, water