We have a data frame from a CSV file. The data frame `DF` has columns that contain observed values and a column (`VaR2`) that contains the date at which a measurement has been taken. If the date was not recorded, the CSV file contains the value `NA`, for missing data. Var1 Var2 10 2010/01/01 20 NA 30 2010/03/01 We would like to use the subset command to define a new data frame `new_DF` such that it only contains rows that have an `NA'` value from the column (`VaR2`). In the example given, only Row 2 will be contained in the new `DF`. The command new_DF