FORco is a company incorporated in foreign country F$.$ FORco sends a sales employee to meet with a U$.$S$.$ customer at the Miami airport to sign a purchase order that will bind FORco only if approved by FORco$$s management in country F$.$ The U$.$S$.$ customer signs the purchase order at the Miami airport and FORco$$s employee goes back home to country F the same day. Which of the following is true?

a$.$ If country F has a treaty with the U$.$S$.,$ FORco has a permanent establishment.

b$.$ If country F does not have a treaty with the U$.$S$.,$ FORco is conducting a trade or business in the US and may have effectively connected income $($ECI$)$ subject to US taxation.

c$.$ If country F has a treaty with the U$.$S$.,$ the transaction is considered a U$.$S$.$ trade or business, but the ECI may be subject to taxation at the lower treaty rates.

d$.$ FORco has a permanent establishment independently of whether there is a treaty or not.