For this case we have by definition, that the volume of a cone is given by:

Where:
A: It is the cone radius
h: It's the height
They tell us that the diameter is 21 m, then the radius is half the diameter, that is: 10.5m. The height is 4m. Substituting the data:
.
Finally, the volume of the cube is
ANswer:
Option B
Answer:
The principal square root of -4 is 2i.
Step-by-step explanation:
= 2i
We have the following steps to get the answer:
Applying radical rule 
We get 
As per imaginary rule we know that 
= 
Now 
Hence, the answer is 2i.
Ok what is it that u need