Points M and N are the midpoints of sides PA and PB of $$ \triangle PAB $$. 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 MN
(b) the perimeter of $$ \triangle PAB $$
(c) the area of $$ \triangle PAB $$
(d) the area of trapezoid ABNM
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