Select the code that performs the following task: from the NHANES data, just pick out the age variable, then from the subset of those who are 18 or older, calculate the IQR (Interquartile Range).
a) age <- NHANES$age; subset(age >= 18); IQR(subset)
b) age <- NHANES$age; subset(age >= 18); IQR(subset(age >= 18))
c) age <- NHANES$age; subset(age < 18); IQR(subset)
d) age <- NHANES$age; subset(age < 18); IQR(subset(age >= 18))
