Answer the following question given this paragraph: 

Ion drives have low thrust but high efficiency. They have already been used on several space missions, including NASA’s Deep Space 1spacecraft and Japan’sHayabusa asteroid sampling probe. Source: Photo courtesy of NASA,http://commons. wikimedia. org/wiki/File:Ion_Engine_Test_Firing_-_GPN2000-000482.


Q: Ion drives have low thrust but high what?

 Choices:

- power

- acceleration

- tolerance

- efficiency

A:
efficiency
