Sort by x$1,$ then by x$2,$ and then by x$3,$ all in ascending order. Note that all variables are sorted simultaneously. After the variables have been sorted, what is the first observation for x$1,$ x$2,$ and x$3?$

Sort by x$1$ in descending order, then by x$2$ in descending order, and then by x$3$ in ascending order. Note that all variables are sorted simultaneously. After the variables have been sorted, what is the first observation for x$1,$ x$2,$ and x$3?$

How many missing values are there in x$1,$ x$2,$ and x$3?$