If a particular set of data is approximately normally distributed so according to the empirical rule approximatly (chose the best answer)

Group of answer choices

a. about 68% of observations would fall within plus or minus 1 standard deviation around the mean

b. about 95% of observations would fall within plus or minus 2 standard deviation around the mean

c. about 99.7% of observations would fall within plus or minus 3 standard deviation around the mean

d. All of the above are true