she has a choice between two activities: answering $200$

telephone calls per hour or responding to $400$ emails per hour. What is the opportunity cost of responding to $400$ phone calls?

c$.$ Deirdre has a choice between writing one more book this year or five more articles this year. What is the opportunity cost of writing half of a book this year, in terms of articles?

$400$ phone calls cost email$($s$)$