Let $$a_{1}, a_{2}.........a_{7}$$ be arbitrary different positive integers taking one value at a time from 1,2,........7. The number $$(a_{1}-1) (a_{2}-2)......(a_{7}-7)$$

A

is odd if non zero

B

is necessarily 0

C

is even if non-zero

D

is always $$\leq 0$$