C) hot air balloons use convection to rise
when air under the envelope is heated, it travels upward and it is less dense as compared to the air in the surrounding . This difference in density values of air inside and outside creates force of buoyancy which make the balloon to get lifted. since here the air is being heated and it is transporting the thermal energy with it, this process is called convection. hence we say that hot air balloons use convection to rise.