Answer the following question given this paragraph: 

Electromotive Force You can think of many different types of voltage sources. Batteries themselves come in many varieties. There are many types of mechanical/electrical generators, driven by many different energy sources, ranging from nuclear to wind. Solar cells create voltages directly from light, while thermoelectric devices create voltage from temperature differences.


Q: Solar cells create voltages directly from light, while thermoelectric devices create voltage from differences in what?

 Choices:

- oxygen

- wire width

- wattage

- temperature

A:
temperature
