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