Points $M$ and $N$ are the midpoints of sides $PA$ and $PB$ of $△PAB$. As $P$ moves along a line that is parallel to side $AB$, how many of the four quantities listed below change?
(a) the length of the segment $PA$
(b) the ratio of the area of $△PMN$ to the area of $△PAB$
(c) the measure of $\angle APB$
(d) the distance from $M$ to the line $AB$