What is the figure of speech use

Reading Comprehension ：Read the quoted parts carefully and answer the questions in English.I wandered lonely as a cloud
That floats on high o'er vales and hills,
When all at once I saw a crowd,
A host, of golden daffodils;
Beside the lake, beneath the trees,
Fluttering and dancing in the breeze.

What is the figure of speech used in the first line?
【正确答案】：

SIMILE