State one main difference between $AC$ and $DC$. Why is $AC$ preferred over $DC$ for long-range transmission of electric power? Name one source each of $DC$ and $AC$.

(N/A) The main difference is that $DC$ (Direct Current) flows in only one direction,whereas $AC$ (Alternating Current) reverses its direction periodically.
$AC$ is preferred for long-range transmission because it can be stepped up to high voltages using a transformer,which significantly reduces energy loss due to heat ($I^2R$ loss) in the transmission lines.
Source of $AC$: $AC$ generator (or alternator).
Source of $DC$: $DC$ generator,battery,or cell.