Excitation contraction coupling in cardiac muscle

What is the primary role of tropomyosin in muscle contraction and how is it regulated?

1: Tropomyosin binds Ca 2+ and directly facilitates actin-myosin cross-bridging.
2: Tropomyosin blocks myosin-binding sites on actin, preventing cross-bridge formation until shifted by troponin C.
3: Tropomyosin acts as a Ca 2+ channel on the sarcoplasmic reticulum.
4: Tropomyosin stabilizes actin filaments without any regulatory function.