A quality analyst wants to construct a control chart for determining whether three machines, all producing the same product, are in control with regard to a particular quality variable. Accordingly, he sampled four units of output from each machine, with the following results

$\backslash$table$[[$Machine$,$Measurements$],[$#$1,17,15,15,17],[$#$2,16,25,18,25],[$#$3,23,24,23,22]]$

What is the sample mean for machine #$1?$

$23$

$16$

$21$

$17$

$15$