AMC 10 2000 Problem 5

Points M and N are the midpoints of sides P A and P B of \triangle P A B. As P moves along a line that is parallel to side A B, how many of the four quantities listed below change?
(a) the length of the segment M N
(b) the perimeter of \triangle P A B
(c) the area of \triangle P A B
(d) the area of trapezoid A B N M

Answer Choices
A. 0
B. 1
C. 2
D. 3
E. 4