The Distribution of a Sample of the Outside Diameters of PVC

The distribution of a sample of the outside diameters of PVC gas pipes approximates a symmetrical,
Bell-shaped distribution. The arithmetic mean is 14.0 cm, and the standard deviation is 0.1 cm. About
68 percent of the outside diameters lie between what two amounts?

A) 13.5 and 14.5 cm
B) 13.0 and 15.0 cm
C) 13.9 and 14.1 cm
D) 13.8 and 14.2 cm