THOSE WHO ARE RIDING A MOTORBIKE ARE NOT ALLOWED TO TAKE OFF THEIR...

17. Those who are riding a motorbike are not allowed to take off their helmet.

a. Those who are wearing a helmet are not allowed to ride a motorbike.

b. It is the helmet that one needn't wear when he is riding a motorbike

c. You should not wear a helmet when you are riding a motorbike.

d. People must never take their helmet off while they are riding a motorbike.