B) You have to assume an initial quantity of whatever the "stuff" is you are measuring.
C) You have to assume there wasn't anything that occurred to alter the decay rate.
You can also think of it like "using an hourglass. You can use the hourglass to tell time if you know several things: the amount of sand in the top of the hourglass when it started flowing, the rate that the sand flows through the hole in the middle, and that the quantity of sand in each chamber has not been tampered with. If any of these three conditions is not accurately known, the hourglass will give an inaccurate measure of time."
A) You have to assume a constant decay rate.
B) You have to assume an initial quantity of whatever the "stuff" is you are measuring.
C) You have to assume there wasn't anything that occurred to alter the decay rate.
You can also think of it like "using an hourglass. You can use the hourglass to tell time if you know several things: the amount of sand in the top of the hourglass when it started flowing, the rate that the sand flows through the hole in the middle, and that the quantity of sand in each chamber has not been tampered with. If any of these three conditions is not accurately known, the hourglass will give an inaccurate measure of time."
(That last part in quotes taken from Ken Ham's website here: http://www.answersingenesis.org/articles/ee2/dating-methods )