A plumb bob is a tool used in construction to determine a vertical reference line. It consists of a weight hanging from a string, aligning itself due to gravity to provide a perfectly vertical reference. The primary function of a plumb bob is to check vertical alignment of structures. ;
A plumb bob is a tool used to determine a vertical line by hanging a weighted point from a string. Its main function is to check vertical alignment in construction projects. Therefore, the correct answer is option D : Checking to make sure that a line is perfectly vertical.
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