Re: In sequence of 9 distinct numbers {a1,a2,a3................a9}, nth

an = an1 + b

a2 = a1+b
a3 = a2+b
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a9 = a8+b

From S1:

a1 = 16
a2 = 16+b
a9 = a8+b
The value of the constant b is unknown. Hence INSUFFICIENT.

From S2:

a5 = 0
a4 = -b
a3 = -2b
a2 = -3b
a1 = -4b
a6 = b
a7 = 2b
a8 = 3b
a9 = 4b
So, Irrespective of the value of b (whether positive of neagtaive), there will be 4 negative values.
If b is +ve, then a1-a4 will be negative
If b is -ve, then a6-a9 will be negative
SUFFICIENT.

B is the answer.