No!
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Yes but phrased differently
Yes, technically a curved line can be a line segment, a line in general is something that can be curved or straight
A line of best-fit.
Because the "best fit" line is usually required to be a straight line, but the data points are not all on one straight line. (If they were, then the best-fit line would be a real no-brainer.)
Finding the line of best fit is called linear regression.