The centre distance between two meshing involute gears is equal to

(a) sum of base circle radii/cosɸ

(b) difference of base circle radii/cosɸ

(c) sum of pitch circle radii/cosɸ

(d) difference of pitch circle radii/cosɸ

I had been asked this question in an interview for internship.

Enquiry is from Comparison Between Involute and Cycloidal Gears topic in section Toothed Gearing & Gear Trains of Machine Kinematics