If you don’t know the answer, don ‘ t be afraid_______so.

The train will arrive at the station in half an hourWho ______for us?—-I am sure Dad will be standing there when we get off the train

Three weeks having gone by, the cause of the accident _____ unknown to the public.[ ]

See the flags on top of the building? That was we did this morning.

下列各句中，表达得体的一句是