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

### Instruction:
What is the highest production code of the episodes having a US viewership of exactly 2.3?

### Inputs:
table_name: table, columns: ['No in. series', 'No in. season', 'Title', 'Original air date', 'Production Code', 'U.S. viewers (millions)']

### Response:
SELECT MAX Production Code FROM table WHERE U.S. viewers (millions) = 2.3