Which of the following elements is used to add new features to an RSS feed without changing the core specifications?

The <channel> element of an RSS feed may contain each of the following optional elements only once. Which of the following elements is an exception to the above statement?

Which of the following elements defines the last-modified date of the content of a feed?

Analyze the code below:
1. <?xml version="1.0" encoding="ISO-8859-1" ?>
2. <rss version="2.0">
3. <channel>
4. <title>Home Page</title>
5. <link>http://www.expertrating.com</link>
6. <description>Free web building tutorials</description>
7. <copyright>Copyright</copyright>
8. <item>
9. <title>RSS Tutorial</title>
10. <link>http://www.expertrating.com/rss</link>
11. <description>New RSS tutorial</description>
12. </item>
13. </channel>
14. <channel>
15. <item>
16. <title>XML Tutorial</title>
17. <link>http://www.expertrating.com/xml</link>
18. <description>New XML tutorial</description>
19. </item>
20. </channel>
21.</rss>
What is the error in the above code?

Which of the following is the correct syntax for writing a comment in an RSS feed?