Examine the built-in dataset co2. Which of the following is true: *
- co2 is tidy data: it has one year for each row
- co2 is not tidy: we need at least one column with a character vector.
- co2 is not tidy: it is a matrix instead of a data frame.
- co2 is not tidy: to be tidy we would have to wrangle it to have three columns (year, month and value), then each co2 observation would have a row.