In a closed system for the reaction: A (g) + 2B (g) → AB₂ (g) The rate of the reaction increases when there is A) an increase in pressure
B) an increase in the reaction vessel
C) a decrease in the concentration of A
D) an increase in surface area of A

The answer is c - the principal of original lateral continuity. 

The population of unionid mussels decreased after zebra mussels were introduced. zebra mussels competed with the unionid mussels for food. the zebra mussels were able to displace the unionid mussels, allowing the zebra mussel population to increase  over the years. the unionid mussel population decreased because  of lack of food.