TMV is not an additional hazard for smokers because the humans are not within the range of hosts of the TMV virus and so the humans cannot be infected. The Tobacco Mosaic Virus is actually a single stranded RNA virus that infects a wide variety of plants and especially the tobacco plants. The main signal is the discoloration of the leaves of the plants.
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The number of covalent bond an element can form depends on the number of electrons it has in its outermost shell (valence electrons). Carbon has four valence electrons, therefore, it can form maximum of four covalent bonds in an organic compound.