Expression for radio horizon in km is ____

• A. \(4.12(\sqrt{h_t}+\sqrt{h_r})\)
• B. \(4.12(\sqrt{h_t}-\sqrt{h_r})\)
• C. \(3.56(\sqrt{h_t}+\sqrt{h_r})\)
• D. \(3.56(\sqrt{h_t}-\sqrt{h_r})\)