Does Pro Sites require Paypal Express Checkout or Pro?

I need some clarification please.

I currently have a Paypal Business Standard account.

The description of "Paypal Express/Pro" on the "Choose a Gateway" option in Pro Sites refers to "enabling the optional Paypal Pro..."

My confusion surrounds the use of "optional."

Do I have to upgrade my Paypal account to either Paypal Express or Paypal Pro in order to process orders through Paypal with Pro Sites, or can I use my current Paypal Business Standard account?

Thanks for your help!