Step-by-step explanation:
the lateral area of the cone is, well, the lateral area.
the total surface area of the cone is the lateral area plus the area of base circle.
so, the difference between both is the area of the base circle :
120pi - 70pi = 50pi
the area of a circle is
pi×radius²
and that is in our case
pi×radius² = 50pi
radius² = 50
radius = sqrt(50) = 7.071067812... ≈ 7.1 units
therefore, C is the right answer.
