Smart phones and Smart watches are same functions due to?

What is the primary difference between the header section of a document and the body? a. the body is displayed on the webpage and the header is not. b. the header is displayed on the webpage and the body is not. c. the tag for the body is self-closing, but the tags for the headers must be closed. d. the tag for the header is self closing, but the tag for the body must be closed.

Worth 50 points answer them bc i am not sure if i am wrong

What are "open-loop" and "closed-loop" systems