It allowed Missouri and then Maine (which also applied for statehood) to become states. A lot of states were on the edge on wether to admit or not allow a state that allows slavery into the states. In the end - both were admitted and became states. However, Missouri was left as a slave state and Maine was set as a free state.
The clause is really a state's rights clause. A state may not pass a law that makes something illegal that was legal before the law was passed. In other words if a state suddenly passed a law that said it is no longer legal to park your car in front of any government building, but it was legal to do so yesterday, the police cannot come to your door and issue a ticket for parking in front of a government building because you did it yesterday.
The constitution actually uses the phrase ex post facto law in Article 1 Section 10 Clause 1.
I believe the answer is B
Answer:COTTON PRODUCTION
Explanation: slave trading and cotton production was the leading industry iss you had 100 slaves you were wealthier then the person with 50 so more slaves is more cotton thats picked faster then the person with less slaves