Consider the model for this assignment problem.

Min $10$x$1$A $+12$x$1$B $+15$x$1$C $+11$x$2$A $+14$x$2$B $+19$x$2$C

$+18$x$3$A $+21$x$3$B $+23$x$3$C $+15$x$4$A $+20$x$4$B $+26$x$4$C

s$.$t$.$ x$1$A $+$ x$1$B $+$ x$1$C $+$ x$1$D $=1$

x$2$A $+$ x$2$B $+$ x$2$C $+$ x$2$D $=1$

x$3$A $+$ x$3$B $+$ x$3$C $+$ x$3$D $=1$

x$4$A $+$ x$4$B $+$ x$4$C $+$ x$4$D $=1$

x$1$A $+$ x$2$A $+$ x$3$A $+$ x$4$A $=1$

x$1$B $+$ x $2$B $+$ x$3$B $+$ x$4$B $=1$

x$1$C $+$ x $2$C $+$ x$3$C $+$ x$4$C $=1$

How many projects are in this assignment problem?

Group of answer choices

$5$

$1$

$7$

$2$

$4$

$6$

$3$