Which sentence contains the time-form error in the past-obligation contrast between must and had to?
When the lift failed yesterday, guests had to climb six flights of stairs.
After the road closure last week, deliveries had to approach from the north.
During last Saturday's storm, two observers must remain at the weather station.✓پاسخ درست
Because the lock jammed this morning, the librarian had to open a side entrance.
توضیح
The weather-station sentence is wrong because must remain does not mark the requirement as past. Had to remain is needed for the obligation created by the storm that ended last Saturday.