101.4a
If an effect has each player choose a card in a hidden zone, such as their hand or library, those cards may remain face down as they’re chosen. However, each player must clearly indicate which face-down card they are choosing.
If an effect has each player choose a card in a hidden zone, such as their hand or library, those cards may remain face down as they’re chosen. However, each player must clearly indicate which face-down card they are choosing.