G9C09

In free space, how does the gain of two three-element, horizontally polarized Yagi antennas spaced vertically 1/2 wavelength apart typically compare to the gain of a single three-element Yagi?

1. AApproximately 1.5 dB higher
2. BApproximately 3 dB higher
3. CApproximately 6 dB higher
4. DApproximately 9 dB higher

Pick an answer to check it.