Radioisotope Thermoelectric Generators (RTGs) are lightweight, compact spacecraft power systems that are extraordinarily reliable. RTGs provide electrical power using heat from the natural radioactive decay of plutonium-238, in the form of plutonium oxide. The large difference in temperature between this hot fuel and the cold environment of space is applied across special solid-state metallic junctions called thermocouples, which generates an electrical current using no moving parts. Elimination tehchnique could have been used if statement 3 was difficult especially because of its second part. Statement 1 is not correct as the principles are different. Thus, options (a), (c) and (d) are incorrect. Statement 2 is correct as its use in the spacecrafts has been its primary applications. NASA has been using the technology since the 1960s. Statement 3 is correct: Pu-238 is used due to its low half-life.