Sequences P and Q are given as follows:
P: 12, 9, 6, 3, …
Q: −4, 0, 4, 8, …
Which one of the following statements about their common differences is correct?
a) Both P and Q have positive common differences
b) P has negative common difference and Q has positive common difference
c) P has positive common difference and Q has negative common difference
d) P has common difference 3 and Q has common difference 4