The field strength due to space wave propagation is E=____

• A. \(\frac{4πh_t h_r}{λd^2}E_0\)
• B. \(\frac{2πh_t h_r}{λd^2}E_0\)
• C. \(\frac{4πh_t h_r}{λd}E_0\)
• D. \(\frac{4πh_t h_r}{λd^4}E_0\)