It does not matter whether or not you have a d or a c to use for an unknown number, because you do not know what that number is. The letter does not abbreviate something, it's just a symbol to fill in the place of the missing number.<span />
Answer:
(b)
or 
Step-by-step explanation:
Given

See attachment for complete question
Required
Determine the volume of the cone
The volume of a square pyramid is:

Where
a = base dimension
From the attachment, the base dimension of the square pyramid is 2r.
So:

The volume becomes;

To calculate the volume of the cone, we simply multiply the given ratio and the volume of the prism.
So, we have:

![V_2 = \frac{\pi}{4} [ \frac{(2r)^2h}{3}]](https://tex.z-dn.net/?f=V_2%20%3D%20%5Cfrac%7B%5Cpi%7D%7B4%7D%20%5B%20%5Cfrac%7B%282r%29%5E2h%7D%7B3%7D%5D)

Open bracket;

Cancel out 4

The above can be written as:


So, we have:
![V_2 = \frac{\pi}{4} [ \frac{(2r)^2h}{3}]](https://tex.z-dn.net/?f=V_2%20%3D%20%5Cfrac%7B%5Cpi%7D%7B4%7D%20%5B%20%5Cfrac%7B%282r%29%5E2h%7D%7B3%7D%5D)
or

Answer:
c
Step-by-step explanation:
If you didn't file taxes by the end of April 15, you will have to face penalties.
Hope this helps!
Is there any more details? Is the triangle an isosceles? Equilateral?