Answer:
A. to determine what you already know about the subject
C. to let your mind run loose and come up with fresh and creative ideas.
Step-by-step explanation:
"Brainstorming" is a technique that is being used for group activities in order to generate spontaneous solutions and ideas to specific topics. In order to do this, the members will contribute their own ideas without being criticized. This means that brainstorming tries to determine what the members already know about the subject. This makes choice A as one of the purposes.
The members are allowed to think freely and let their mind run loose. Even wild ideas are welcomed in order to drive new perspectives. This makes choice C as one of the purposes.