Which two factors can cause convection currents?

Convection currents are driven by the motion of fluids (which can be liquids or gases) due to differences in temperature and density within that fluid. The correct choice highlights the role of unequal heating of the Earth's surface and thermal expansion, which are fundamental in understanding how convection works.

When the Earth's surface is heated unevenly by the sun, some areas become warmer than others. This temperature difference leads to variations in density; warmer areas tend to have less dense air or fluid, causing them to rise, while cooler areas have denser air or fluid, causing that to sink. This continuous cycle creates convection currents.

Thermal expansion plays a crucial role in this process, as materials expand when heated and contract when cooled. As warm air or water rises, it allows cooler, denser air or water to move in to take its place, thus forming a current.

The other options may reference related concepts such as air pressure or weather, but they do not encapsulate the primary mechanisms that generate convection currents as clearly as the correct answer does.