They _ once at a wedding many years ago

单选题

_____ they arrived there, they started to work at once.

答案解析

单选题

They _ once at a wedding many years ago

答案解析

多选题

______ these potential problems, two-way radios are preferable as they are extremely reliable for short distances and can broadcast to several people at once.

答案解析

判断题

数据集成同步不仅支持at least once，也支持exact once。

答案解析

单选题

I demand that one of them___there at once. (2.0)

答案解析