Which method is valid for proving that two circles are similar?
a- Calculate the ratio of the diameter to the radius for each circle and show that they are equal.
b- Calculate the ratio of degrees to the circumference for each circle and show that they are equal.
c- Calculate the ratio of the área to the circumference for each circle and show that they are equal.
d- Calculate the ratio of the diameter to the circumference for each and show that they are equal.