If you had a pizza with crust thickness 'a' and radius 'z', what's the volume of the pizza?

Can you find the key in the picture below?

What is the word or phrase?

I have no doors but I have keys, I have no rooms but I have space, you can enter but you can’t leave! What am I?

I am white when I am dirty, and black when I am clean. What am I?

In the Chess Board picture below white army is arranged. You need to add a black army on the board such that no piece is under any threat.
Note: Army comprised of 1 king, 1 queen, 2 rooks, 2 bishops, 2 knights, and 8 pawns.

What does man love more than life, hate more than death or mortal strife; That which contented men desire; the poor have, the rich require; the miser spends, the spendthrift saves, and all men carry to their graves?

Find the next number in the series.
1 10 24 43 67 ?

What does the following BrainBat signifies?

ABCDEFGHIJKLMNOPQRSTUVWYZ

QQQQQQQQQQQQQQQQQQQQQQQQQ.

It's a protector.
It sits on a bridge.
One individual can see directly through it, while others wonder what it hides.

What is it?