D, light can travel in the air or a vacuum and the speed is constant whether the light source is stationary or moving.
<span>The atmosphere of Venus is mostly carbon dioxide, 96.5% by volume. Most of the remaining 3.5% is nitrogen. Early evidence pointed to the sulfuric acid content in theatmosphere, but we now know that that is a rather minor constituent of theatmosphere.
T</span>he atmosphere of Neptune consists mainly of hydrogen and helium, but about 2.5-3% of the atmosphere is methane. Like Uranus, clouds in Neptune's atmosphere<span> are composed of crystals of methane.</span>
