To determine the radius needed for the pole to fit perfectly on the diagonal of the cylindrical container, we can visualize the problem as a right triangle where:
- One leg is the height of the container (8 ft),
- The other leg is the diameter of the base of the cylinder (which is double the radius),
- The hypotenuse is the length of the pole (10 ft).
Let's denote the radius of the container as \( r \). Therefore, the diameter would be \( 2r \).
According to the Pythagorean theorem:
\[ (10)^2 = (8)^2 + (2r)^2 \]
Calculating the squares:
\[ 100 = 64 + 4r^2 \]
Subtract 64 from both sides:
\[ 36 = 4r^2 \]
Now, divide both sides by 4:
\[ 9 = r^2 \]
Taking the square root of both sides gives:
\[ r = 3 \]
Thus, the radius needed for the pole to fit perfectly on the diagonal is 3.0 ft.
Therefore, the correct answer is 3.0 ft.