This is a test of commonsense with single/multi-choices. Complete the next sentence:

How to fit a riding helmet
Measure your head.
Helmet sizes correspond to the circumference of your head measured at its widest point. To find your size, use a flexible measuring tape and wrap it around your head about 1 inch/2.5 cm above your eyebrows.

Available choices:
[a]. Subtract 1 inch to get the measurement for the size of your head. It should fit against your head snugly, with a small amount of elasticity at the crown of your head.
[b]. Then, place a tape measure up and down your head, drawing it in a loop first. Then, measure the width of your head using a flat-head fitter tape.
[c]. This will help you adjust your helmet length accordingly. If the helmet looks patchy, the measurements might be adjusted for the seams, not for the face.
[d]. Make sure it stays flat to get an accurate measurement. Note the result in both inches and centimeters, and write it down to take to the store with you.
The answer is:
[d].