R code question

Charlie had a dataframe called df which had a column called values containing several numbers. Select the R code statements below that would calculate the mean of this column. Select one: a. sum(df$values)/length(df$values) b. mean (values) c. all of the above. d. sum(df$values)row (df$values) O What does the argument replace-TRUE ensure when executing the sample() function on a vector? Select one: a. Each unit in the population has only one chance of being selected in the sample. b. Each unit in the population has more than one chance of being selected in the sample c. Each unit taken from the population for the sample is replaced with an entirely new unit in the population. d. None of the above