Why does cytosine always have to match with guanine?

Which gemstones are cabochon cut aquamarines turquoise diamonds opals star sapphires? Can you replace a 20 amp fuse in your car with a 25 amp due to the fuse keep blowing? How doornob used? What materials are used in the wispa packaging? What is clangomerate used for? How was the periodic table first organized atomic number or atomic mass? Elements average atomic mass? What conditions favor folding and faulting? The angle of Q represents? What 2 countries r located entirely within 0 degrees and 10 degrees north latitude and 50 and 60 west longitude? Why doesn't ocean water ever get diluted by fresh water rivers running into the oceans? Where is the sacral foramina? What happens to plant and animal life when humans make a major change in the environment? Why is the Nervis system better suited to control a persons movement then the endocrine system? Is helium in neon signs? How is the internal of the Earth hot? Why some scholars define geography as an imposter subject? In terms of atomic structure of matter what is meant by electric current? The interrelationship of the mind and the body is called the mind-body? Is this statement true or false rivers cite more V shaped valleys while glaciers carve out valleys that are U-shaped?