The leading explanation for the existence of spiral arms are: Group of answer choices shock waves from cyclic supernovae explosions. shock wave from the quasar-like eruption of the galactic core. tidal streams of small galaxies being torn up by our monster Galaxy. passages of massive globular clusters through the galactic plane. passages of spiral density waves through the interstellar medium.

Why is rain likely when warm, moisture-laden air meets cold air? a) the lighter warm air will rise and cool down, causing condensation and rain. b) the cold air moves faster and pushes the warm air away, causing condensation and rain. c) the moisture in the warm air condenses on contact with the cold air, causing rain to fall. d) the cold air mixes with the warm air, reducing its temperature causing moisture to condense.