music: 80's Chart Toppers: Name The Artist: We Belong?

Questions

Which composer wrote a concerto for three pianos and orchestra?

In listings of Mozart's works, the title is often followed bY a "K."and a number. What's the "K" stand for?

Who first published the admonition "please do not shoot the pianist.He is doing his best"?

What was the first name of Joplin, composer of "The Entertainer"and other rags?

What was Albert Ketelbey's real name?