We can determine that by doing an experiment. We can put each aqua plant (same species, same size) into bottles of full water, then shine different colors of lights on each bottle. The faster the rate of photosynthesis, the more oxygen the plant release. Therfore which bottle has the less water at the end, the better the light it is.
Trees are not at fault for releasing greenhouse gas. Rather, they are the natural way of absorbing the gas in our atmosphere and are our primary natural ally.
The propagation velocity of the waves depends on density and elasticity of the medium. Velocity tends to increase with depth and ranges from approximately 2 to 8 km/s in the Earth's crust up to 13 km/s in the deep mantle.
