Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
What is the series number that had 5.31 million viewers?

### Inputs:
table_name: table, columns: ['Series #', 'Season #', 'Title', 'Directed by', 'Written by', 'Original air date', 'Production code', 'U.S viewers (million)']

### Response:
SELECT MAX Series # FROM table WHERE U.S viewers (million) = 5.31