Points $M$ and $N$ are the midpoints of sides $PA$ and $PB$ of $△PAB$. As $P$ moves along a line that is perpendicular to side $AB$, how many of the four quantities listed below change?
(a) the length of the segment $MN$
(b) the perimeter of $△PAB$
(c) the area of $△PAB$
(d) the area of trapezoid $ABNM$