A retailer's email marketing campaign promoted free shipping for a limited time. The campaign's performance metrics are listed below. Which of the following formulas would the retailer use to calculated the Click$-$to$-$Open $($CTO$)$ rate?

Emails were sent to $500$ target recipients

Emails were not delivered to $20$ recipients because their inboxes were full

Emails were not delivered to $10$ recipients because the addresses were incorrect

$200$ recipients opened the email

$100$ recipients clicked on the call$-$to$-$action $($CTA$)$ in the email to visit the retailer's landing page

$50$ recipients performed the retailer's desired action, which was defined as a product purchase

a$.$

$100/200$

b$.$

$50/100$

c$.$

$100/500$

d$.$

$200/500$