Which alphabet is a part of our body?

Replace the question mark with the correct number in the below-given picture?

Solve the following series:
AZ, GT, MN, __, YB

I have feet, but no legs. Which animal Am I?

Find a 9-digit number, which you will gradually round off starting with units, then tenth, hundred etc., until you get to the last numeral, which you do not round off. The rounding alternates (up, down, up ...). After rounding off 8 times, the final number is 500000000. The original number is commensurable by 6 and 7, all the numbers from 1 to 9 are used, and after rounding four times the sum of the not-rounded numerals equals 24.

What has 13 hearts but no other organs?

Our product and our sum always give the same answer. Who are we?

If nine thousand, nine hundred nine dollars is written as $9,909,
how should twelve thousand, twelve hundred twelve dollars be written?

A four-digit number (not beginning with 0) can be represented by ABCD. There is one number such that ABCD=A^B*C^D, where A^B means A raised to the B power. Can you find it?

I am the fastest animal but cannot climb the tree. What am I?