How many required and optional sub-elements does <channel> contain?

Which of the following hexadecimal references must be used by a
publisher when encoding the "&" character in an RSS feed?

Which of the following is included in an RSS feed for the
modularization of the feed?

Which of the following hexadecimal references must be used by a
publisher when encoding the ">" character in an RSS feed?

Which of the following is true about the <ttl> element of an RSS feed?