Why are light-years more convenient than miles, kilometers, or astronomical units (au) for measuring the distances to stars and galaxies? (select all that apply.) a) because in viewing a distant galaxy, the distance in light-years to the galaxy tells us how far back in time we are viewing the galaxy b) because light-years are the units astronomers have been using since the beginning of modern astronomy and are used only to be consistent with historical convention c) because distances between objects in the universe are very large compared to the distances between objects on earth and in our solar system, and measuring distances between objects in the universe in miles, kilometers, or astronomical units would require using very large numbers d) because only light-years can provide an accurate measurement of how far light travels across the universe