Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Race Name | Circuit | Date | Winning driver | Constructor | Report |
| --- | --- | --- | --- | --- | --- |
| II Race of Champions | Brands Hatch | 12 March | Dan Gurney | Eagle - Weslake | Report |
| I Spring Cup | Oulton Park | 15 April | Jack Brabham | Brabham - Repco | Report |
| XIX BRDC International Trophy | Silverstone | 29 April | Mike Parkes | Ferrari | Report |
| XVI Gran Premio di Siracusa | Syracuse | 21 May | Mike Parkes Ludovico Scarfiotti | Ferrari | Report |
| XIV International Gold Cup | Oulton Park | 16 September | Jack Brabham | Brabham - Repco | Report |
| XV Spanish Grand Prix | Jarama | 12 November | Jim Clark | Lotus - Ford | Report |
Question:
What is the circuit held on 15 april?
Answer:
Oulton Park