Fill in the blank with an appropriate word, phrase, or symbol(s).

According to the empirical rule, in a normal distribution,

a) approximately ________ of the data lie within plus or minus 1 standard deviation of the mean.

b) approximately ________ of the data lie within plus or minus 2 standard deviations of the mean, and

c) approximately ________ of the data lie within plus or minus 3 standard deviations of the mean.