Which of the following statements correctly describe the teachings of the Upanishads?

(A) The Upanishads emphasize the importance of performing rituals meticulously.

(B) The concept of karma and rebirth is a central theme in the Upanishads.

(C) The Upanishads teach about the relationship between the individual soul (Atman) and the universal soul (Brahman).

(D) The primary focus of the Upanishads is on the worship of multiple deities.