The radius of a fluorine atom is about 5 x 10” millimeter. The radius of a strontium atom is about
2 x 10-7 millimeter.
About how many times larger is the radius of a strontium atom than the radius of a fluorine atom?

(n^3 - n)

= n(n-1)(n+1)

since it is divisible by 3 and 2 ,because it is a multiple 3 consecutive number

if, n-1 is odd then n+1 is also odd, then it is a multiple of 9

but if, n-1 is even then n+1 is also even, then it is a multiple of 4

so, largest multiple = max(9*2, 4*3) = max(18, 12) = 18

the answer is b