A. This is true. If 0 < r < 1, then we have decay. For example, if r = 0.5 then we cut each term in half to get successive new terms (10, 5, 2.5, 1.25, etc etc)
B. False. Common differences apply to arithmetic sequences which are not exponential in nature.
C. False. Decay curves slope downward. As you read them from left to right, they will drop down.
D. False. If r > 1, then we have exponential growth. It is effectively the opposite of choice A.
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So in summary, the final answer is choice A
