钢琴家忘记了乐谱，只好即兴演奏了一首曲子。

The pianist forgot the sheet music and had to ___extemporize a piece.

演讲者没有准备讲稿，而是即兴发挥回答了观众的问题。

The speaker didn't prepare a speech but ___extemporized answers to the audience's questions.