Sunlight is made of a mix of colors, but as it travels through the air, the gas molecules scatter light. When the scattering depends on wavelength, shorter wavelengths scatter much more strongly than longer ones. This is Rayleigh scattering. Because blue light has a shorter wavelength, it gets bounced in many directions by air molecules, so the sky looks blue from everywhere you look. Violet light is scattered even more, but there’s less violet reaching us from the Sun and our eyes are more sensitive to blue, so the sky doesn’t appear violet. If larger particles were responsible, a different kind of scattering would happen that tends to produce a whiter, less blue sky (Mie scattering). Diffraction isn’t the main effect here, and fluorescence involves light being absorbed and re-emitted, not just scattered in the atmosphere, so it doesn’t explain the blue sky.

Destructive boundaries happen where tectonic plates collide or one is forced downward into the mantle. This type of movement is described by the term convergent boundary, which is the label for plates moving toward each other and interacting vigorously. In subduction zones, an oceanic plate sinks beneath another plate, creating trenches, volcanic activity, and strong earthquakes; when two continental plates collide, mountains are built. Other boundary types describe different motions: divergent boundaries where plates move apart, and transform boundaries where they slide past one another. A passive boundary isn’t actively tectonically interacting. So the proper name for this situation is a convergent boundary.

After cytokinesis, the cell finishes dividing its cytoplasm and re-enters interphase, starting with the first stage called G1. In G1 the cell grows, carries out normal functions, and checks that conditions are favorable before copying its DNA. Only after G1 does it move into S phase for DNA replication, then G2 to prep for another division. Mitosis, which includes prophase through cytokinesis, has just concluded with cytokinesis, so the next phase is G1.

Lines that run from east to west on a map are lines of latitude, which are also called parallels. They form circles around the globe and lie parallel to the equator, which is why they’re described as parallels. These lines show how far north or south you are from the equator. In contrast, lines that run north to south are longitudes (meridians), which measure east–west position and converge at the poles. The term parallels is the precise description for those east–west lines, making it the best choice.