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

### Instruction:
Which Thickness has a Diameter of 21.9mm?

### Inputs:
table_name: table, columns: ['Year of Issue', 'Description', 'Weight', 'Diameter', 'Thickness', 'Edge']

### Response:
SELECT Thickness FROM table WHERE Diameter = 21.9mm