If there are 21 values in a data set in order from smallest to largest, what is the first quartile of the data set?
AThe mean of the 6th and 7th values
BThe mean of the 5th and 6th values
CThe mean of the 3rd and 4th values
DThe mean of the 4th and 5th values
AThe mean of the 6th and 7th values
BThe mean of the 5th and 6th values
CThe mean of the 3rd and 4th values
DThe mean of the 4th and 5th values