There are two triangles formed from the vertical and horizontal cross-sections of the cone.

Triangle 1: Triangle 2:

Find the height $\"\"$ of the Triangle 1:

Pythagorean theorem:

The square of hypotenuse is equal to the sum of the squares of the other two sides.

$\"\"$

$\"\"$ .

Find the height of the smaller top cone.

$\"\"$ is the height of the smaller top cone.

Since two angles of triangle 1 are congruent to two angles of triangle 2, triangle 1 and triangle 2 are similar triangles , and

$\"\"$ .

Substitute $\"\"$ in $\"\"$ .

$\"\"$

$\"\"$ .

Option B is correct.