"si oui, lesquels" was also given right.
Why "si oui" isn't shortened to "s'oui" in this sentence?
There are 2 vowels one after another.
Si only contracts to s' before il and ils : si il -> s'il, si ils -> s'ils
More generally, I believe « si » would contract with any word starting with an "i," it's just that most of the time, that word is « il » / « ils. »