If the circumference of a circle is doubled, the diameter of the circle
(1) remains the same
(2) increases by 2
(3) is multiplied by 4
(4) is doubled
you know already that the circumference of a circle is calculated by:
, with diameter = 2 * radius.
Plug in the value you know:
PS: I've attached a diagram which helps to memorize the right formula: Cover the variable you want to calculate with your fingertip. The remaining term shows how to calculate the covered value.