Explain why thermal energy from the sun reaches the cups through radiation, instead of conduction or convection.

Explanation:

Heat from the sun cannot reach the earth by conduction or convection because both these processes require a material medium (like solid, liquid or gas) to transfer heat, they cannot take place in empty space (or vacuum). Heat from the sun reaches on the earth by the process of radiation.

r = ρ x length / area  

ρ = r x area / length  

ρ is in ohm-meters  

in basic units, it is m³kg/s³a²  

i think this is it.