A very hard substance is composed of atoms that make a network of strong covalent bonds, with no discrete molecules. It is a poor conductor of electricity

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Explanation:

The covalent network solid is composed of a network of strong covalent bonds rather than discreet molecules.

Covalent network solids are composed of atoms covalently bonded together into a three-dimensional network or layers of two-dimensional networks. Due to the strength of the covalent bonds, covalent network solids have high melting points(Khan Academy).

Examples of covalent network solids are Boron nitride, Silicon dioxide, diamond, graphite etc.

a boiling water reactor does not boil the water. a pressurized water reactor boils the water faster. a pressurized water reactor is heated directly by the core. a pressurized water reactor does not use fission.

in cartography, a contour line (often just called a "contour") joins points of equal elevation (height) above a given level, such as mean sea level. a contour map is a map illustrated with contour lines, for example a topographic map, which thus shows valleys and hills, and the steepness or gentleness of slopes.

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