Which of the following expresses the distance PQ between two points P(x1,y1,z1) and Q(x2,y2,z2) in space?
a) PQ=(x2−x1)2+(y2−y1)2+(z2−z1)2
b) PQ=(x2−x1)2+(y2−y1)2
c) PQ=(x2−x1)2+(y2−y1)2+(z2−z1)2
d) PQ=∣x2−x1∣+∣y2−y1∣+∣z2−z1∣