help!

Which of the following statements is true of the html5 <source> tag?
	 a. It can be nested inside the <audio> tag to point to alternative audio formats
	 b. It can be nested inside the <video> tag to point to alternative video formats
	 c. It must be nested inside the <video> tag to point to the source file
	 d. It must be nested inside the <audio> tag to point to the source file
	 e. there is no <source> tag in html 5