10.14 mentioning the agent with 'by'

When you use the passive, you can of course mention the agent at the end of the clause by using 'by'. But this puts emphasis on the agent, because the end of the clause is an important position, and so you often do this when you want to refer back to the agent in the next clause.

• His best friend was killed by a grenade, which exploded under his car.
• The defending champion, John Pritchard, was beaten by Chris Boardman.
• This view has been challenged by a number of workers.