A Control Zone has to exist to at least:
a. 5 NM from the centre of the airfield or airfields concerned in the direction from where approaches can be made.
b. 20 NM from the centre of the airfield or airfields concerned in the direction from where approaches can be made.
c. 15 NM from the centre of the airfield or airfields concerned in the direction from where approaches can be made.
d. 10 NM from the centre of the airfield or airfields concerned in the direction from where approaches can be made.