This is a geometric sequence.
a = 1000 is the first term
r = 0.4 is the common ratio (since a2/a1 = 400/1000 = 0.4)
a(n) = a*r^(n-1)
a(n) = 1000*0.4^(n-1)
a(n) = 1000*0.4^n*0.4^(-1)
a(n) = 1000*0.4^n*2.5
a(n) = 1000*2.5*0.4^n
a(n) = 2500*(0.4)^n
Answer is choice B
