Complete the negative one-off sentence with could or be able to, using the exact verb in brackets: Despite using the correct code yesterday, Sofia ___ the storage locker. (unlock)

Объяснение

Could not unlock and the equivalent be-able-to forms all report Sofia's failed attempt on that particular occasion. An affirmative phrase such as could unlock would describe ability rather than the stated failure, while changing unlock to another verb would not complete the prescribed phrase.