How to calculate the perimeter of flat figures?

I - The perimeter of a flat figure is the sum of the lengths of its sides.
II - To calculate the perimeter, it is necessary to know the length of all sides of the figure.
III - The perimeter of a triangle whose sides measure 5 cm, 6 cm, and 10 cm is 21 cm.
a) Only I is correct.
b) Only II is correct.
c) Only III is correct.
d) I and II are correct.
e) I, II, and III are correct.