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}) )