You needn’t have apologized for your absence—      , it’s hardly your fault.

They had planned to go for a picnic, but because of the rain they had to stay home_______.

The traffic problems have severely affected economic development. ________, we must spare no efforts to solve them.

Polar bears live_______ on sea ice，which they use as a platform for hunting seals

Flight attendants usually need to arrive at the airport ______ before their flight leaves.