In Grails, which of the following command lines is used to enable the read-write caching mechanism on a collection of instances:
1. cache: 'read-write' // or 'read-only' or 'transactional'
2. cache: 'read' // or 'read-only' or 'transactional'
3. cache: 'write' // or 'write-only' or 'transactional'