Which R function will generate a mean for each row in a numeric matrix X with intermittent missing values?

Suppose numeric vectors X, Y, and Z are all of the same length. Which R command will create a matrix with rows X, Y, Z?

Which R command is used to merge two data sets X and Y by the variable "ID" that includes all records from both data sets?

Which object class can contain multiple unrelated object classes of potentially different sizes?

I want to use the sort() function, and I see in the help file that I my sort "an R object with a class, or a numeric, complex, character, or logical vector." Given this guidance, which is a valid call to sort()?

Which is NOT a valid R function for obtaining the residuals from a simple linear regression model of X predicting Y?

Which of the following statements regarding mixed vector and array arithmetic is true?

Which of the following is NOT one of R's basic vector types?

Which one is the correct expression regarding outer product of two arrays?

Which R code subsets a matrix Y to only those records (rows) where a variable x (in Y) is less than 100?

What R function can be used to tabulate values of categorical variable Y (columns) by categorical variable X (rows), including missing values?