【温故知新】 阅读下列句子，根据提示完成句子并体会其用法 ①Scientists have tried many ways _______(solve) this problem. ②The wind is sometimes strong enough _______(move) sand dunes. ③“__________________(catch) in a sandstorm was a terrible experience, ” he said. ④Traffic moves very slowly because the thick dust makes it difficult _____(see).

to solve

to move

To have been caught

to see