Which one of the following is the next number in the series : 7, 7, 14, 42, 168-----?
A
336
B
504
C
672
D
840
Correct Answer: 840
7 × 1 = 7;7 × 2 = 14; 14 × 3 = 42; 42 × 4 = 168; 168 × 5 = 840