Admob "Loading" and "Showing" places - where?

hi everybody.

you can load an e.g. interstitial.
than you can display it.
ok. i know how to work with these functions.

My question:

lets say you have a game with 1 startpage and 3 scenes.
the interstitial should be shown on each scene after a special event.

what are the correct places for the loading and showing functions?

a) “loading”-function ONCE e.g. in the startpage.
“showing”-function then in EACH scene 1, 2, 3 ?

or

b) “Loading”-function AND “showing”-function
in EACH scene 1, 2, 3 ?

thanks for advise.

and…
happy new year!