How many 4-digit positive integers are there, where each digit is positive, and no 3 adjacent digits are same? Choose one:
A) 1236
B) 3528
C) 4096
D) 4608
E) 6561

Rationale: first digit . . 9 posibilities second digit , 8 possibilities third digit , 7 possibilities fourth digit , 7 possibilities . 9 * 8 * 7 * 7 = 3528 . b
Answer: B