Which of the following statements are true concerningelectromagnetic waves in a vacuum? Check all that apply

	Electromagnetic waves travel in a vacuum with a definite andunchanging speed.
	Electromagnetic waves can travel in a vacuum only because amedium prohibits the forward propagation of the electric andmagnetic fields.
	When electromagnetic waves travel in vacuum, there is adefinite, constant ratio between the magnitudes of the electric andmagnetic fields.
	Electromagnetic waves traveling in a vacuum are longitudinalwaves.