A data set has a mean of 50 and a standard deviation of 10.

Which of the following might possibly be true, and which are impossible?

a. More than 10% of the data values are negative.

b. Only 5% of the data values are greater than 70.

c. More than 20% of the data values are less than 30.

d. Less than 75% of the data values are between 30 and 70.