A drone must fly straight from point P(x1,y1) to point Q(x2,y2) on a coordinate grid. Which of the following expressions gives the distance PQ in units?
a) (x2−x1)2+(y2−y1)2
b) (x2+x1)2+(y2+y1)2
c) (x2−x1)2+(y2−y1)2
d) (x2−x1)+(y2−y1)