Which statement describes a difference between electromagnetic and mechanical waves? a. mechanical waves do not transfer energy, but electromagnetic waves do. b. mechanical waves cannot be longitudinal, but electromagnetic waves can. c. electromagnetic waves cannot move particles, but mechanical waves can. d. electromagnetic waves do not require a medium, but mechanical waves do

The answer is D.

Mechanical waves need a medium to travel, for example, sound is a mechanical wave. You are able to hear other people because the sound wave is travelling trough a fluid -air-. If you were to be in space -vacuum- you would not be able to hear anything. 

The most well known example of an electromagnetic wave is light. As you may have noticed, we're able to get the light from the Sun to Earth; clearly light -electromagnetic wave- is able to travel in vacuum/space (i.e., without a medium).

Efficiency is an action of doing something. for example when you take a test. good efficiency means you did well but bad effeciency or no effiency means you did bad.